Why T-scale is a good idea:

In conditions where space is at a premium, and portability is important, smaller scales have a decisive advantage.  Note here how a T-scale layout can be embedded into a suitcase, coffee table, or other similarly small space:

An additional upside to saving space, is cost.  T-scale layouts may indeed be more costly per square inch, but given the choice between an interesting sceniced T-scale layout and the same track layout built in HO, the most common model railroading scale (1:87), there's a definite advantage to building everything smaller.

A brief survey of layouts auctioned on eBay suggests that the typical sceniced HO layout costs somewhere around $0.64 per square inch of surface space used, and the typical T layout is up to $2.42 per square inch - but given that HO is 5.19 times as large in every dimension, T scale fits 26.9 times as much layout into each square inch.

In this example, we demonstrate how building miniatures in a smaller scale saves money and space; the T-scale version of this layout would cost just under $700, and can fit in a small carrying case and can be shipped and moved anywhere fairly cheaply and easily.  The HO-scale equivalent, however, would cost over $4000, is over 9 feet in width, and would not be easily portable.

Detailing in any one scenic object will, of course, be less in a smaller scale.  A building or tree in 1:450 cannot hold as much detail as a building or tree in 1:87th, but the flip side of this is that there's not really much point in adding certain details to something so small.  A wildflower, a fallen tree branch, a clump of grass, a mouse - these small objects are visible in 1/87th scale and must be sculpted to make the scene believable.  In 1/45oth, however, these small bits of detail are not needed at all because even if you did somehow build them, nobody would be able to see them without a strong magnifying glass! 

To put this in terms of printing on paper, 300 dots per inch is just enough to make a print look pristine and detailed, even when scrutinizing it very closely... this suggests that when viewed as closely as possible, with one's face practically right on top of the miniature, the smallest details the human eye can discern in an object are those which are around a tenth of a millimeter in size.  When viewed from a comfortable viewing distance, that changes to half a millimeter or so...  which, when multiplied by 450, comes out to about 7-8 inches.  In practice, this means that no object smaller than a human head necessarily needs to be modeled at all in a T-scale layout, because under most conditions, nobody will be likely to see it anyway.

This also means that a layout in T-scale or other small scales will require much less time to make "photoreal" than the equivalent in HO-scale.  T-scale scenery is very impressionistic; only the broad strokes of a scene need to be present at all. 

In scales smaller than T-scale, human figures disappear entirely.  While meticulous 1:900 or 1:1000 miniatures are fascinating to view, they do not typically have any people in them.

In summary:  The smallest scales require less time, less money, and less space - and are far more portable.  They also have, at least at the moment, a special "Wow" factor.  People look at a T-gauge train set and can't believe how tiny it is.

...and why T-scale might not be a good idea:

Because these scales are smaller, small details are omitted.  If the rail modeller's desired emphasis is on human figures and details of human-scale scenes rather than the broader landscape, T-scale and smaller scales are not well-suited to that.  If, however, the objective is to create an elaborate and complex landscape and track layout, with minimal use of time, money, and space, small scales may prove beneficial.

It is arguably desirable to concentrate attention on slightly larger scales - like Z scale (1:220) or N scale (1:160) which provide a reasonable balance between the strengths of tiny scales, and the strengths of larger scales.

Z and N scale offer a pleasing balance between human-scale scenes, and broader landscapes, without requiring an inordinate amount of space.  N scale is, perhaps for this reason, one of the most popular model railroading scales - with many trains and scenic products available at this size (though perhaps not quite as many as with HO-scale).

Another downside, and the reason most modelers avoid scales as small as T-scale, is that they can cause eye strain, require good hand-eye coordination, and may be fragile and easily ruined.  Delicate, difficult-to-make scenery objects are probably the single biggest frustration with 1:450th scale.

 Eishindo creates untextured plastic buildings that must either be meticulously painted with a fine-tipped brush or left in flat colors.  Most T-scalers have responded to this by printing building designs onto paper and then folding the paper by hand into the desired shape, but folding a tiny scrap of paper into a building can be an exercise in frustration, and even if completed correctly, is very easily ruined.  Touching a centimeter-tall paper model with your finger is enough to destroy it, and thin balsa wood painted and assembled by hand is little better.

I personally advise the use of a mounted magnifying glass to minimize eye strain, and have launched a product line of 3d-printed, prefabricated buildings as a possible emerging alternative to paper or plastic ones.

T-scale is not for everyone, and due to its sometimes finicky operation, limited number of products available, and particularly small size, has often been dismissed as a 'novelty' scale - much like HT/TY scale (1:900)... but the options available for T-scale modelers have improved steadily over time.

 This intricate scale has grown in popularity over time, expanding rapidly in popularity since 2008, but some concerns remain, including the fact that a single manufacturer, Eishindo, is responsible for creating T-scale trains and track.  Should Eishindo suffer financial collapse, the scale could die with it. 

Interest in this scale seems to be waning slightly as of May 2011 - if this blog post is accurate:

Worried about T-gauge There have been some signs of a lull in interest in T-scale this year.

I think the 2011 Japan earthquake may have been a factor - it might be causing problems for Eishindo and disruptions in the T-scale market in Japan.  With so much structural damage in that area, it is no wonder that hobbies and nonessentials like model railroading are taking a hit.  Meanwhile, fears of a double-dip recession in the United States and economic problems in Europe, have affected spending there.  Simply put, economic prospects look bad all over the place, and that probably is harming the model railroading hobby, especially niche markets like T-scale.  There's also the fact that model railroading routinely sees a lull in activity during the summer.

 T-scale also has seen a hostile backlash from those older miniature enthusiasts who view the scale as too small and too impractical to be a 'real' model railroading scale.  (Of course, the same has been said of Z and N scale in the past, and now they're widely regarded as legitimate scales.)

I have checked my web statistics, however, and find a massive, steadily growing number of visitors to this website each month... I suspect there's a still-growing interest in the scale, and that even though fewer people are building T-scale layouts right now, this has more to do with a bad economy and lack of disposable income than lack of enthusiasm.