well, TT size is super important to someone that knows sizing well and is tuned in to the differences in stem length, saddle position, and all of the other micro-adjustments that can be made to make for a more efficient climb, sprint, etc etc etc etc. for a guy that doesn't know what size his bike even is i don't think he'd be able to discern the difference between a few cm's of TT length. (no diss at all intended). as a safety precaution, one of the things most concerned is if you ever had to bail or anything, you would really need that 2 inches of play between toptube and groin. more so than, say, an extra 1-2cm's of TT length.
the 2 inch TT/groin sizing method is rudimentary, of course, but a pretty good estimate when getting the ballpark size. a fitting downtube length is almost always going to have a nice proportionate toptube length to match, and a short or long tt can always be compensated for by stem length, bars, saddle.