I should know better than to post on a technical subject without reviewing my old texts, but here goes anyway.
The T-tail does have a couple of advantages, and also some disadvantages. The thing to remember is that 99% of the design/aero/engineering decisions made in designing an
aircraft are compromises. The T-tail is used when its advantage outweighs it's disadvantages. I have also observed that there are very few commercial aircraft that use the T-tail and the ones that do have a fairly obvious reason for doing so. Aircraft that were intended to be sold to non commercial users seem to use the T-tail more as a device to give them market differentiation.
With that out of the way, let me attempt to give some concrete answers.
Advantages: it gets the tail out of turbulent air, it allows for a longer tail arm (allowing smaller surface area, and therefore, less drag), it has some potential for improved stall recovery, and it looks cool.
Disadvantages: reduced control authority for prop planes (it's out of the propwash) , increased complexity for the control linkages, heavier vertical stab required because vertical stab must carry/transfer the pitch loads to the tailcone, heavier vertical stab required becouse the vert. stab must also transfer twisting loads to the tailcone, heavier tailcone required because the aero forces developed twist the tail (especially in the case of a high G rolling pullup)
T-tails get used most often when other design criteria force the designer to use them. For example, the Learjet was designed around a pre-existing wing and that wing had no provision for mounting engines, therefore they had to mount the engines on the tail and therefore the T-tail worked best.
The DC9 would be another example of engine placement dictating tail style. The 727 uses the T-tail for somewhat the same reason, it was easier to fit to the top of the tail and they already had to make the vertical stab extra wide in order to accomodate the third engine. The ATR turboprop aircraft use a T-tail partly because it allows the horizontal tail to be installed slightly farther away from aerodynamic center of the wing and allows for a slighty smaller tail (the tail volume issue) I'm not totally certain, but I think the B1900 uses a T-tail for the simple reason that the B200 used one and the engineers determined that they would be able to get what they wanted for the 1900 by modifying that existing tail.( if they had used the B100/B99 tail, they wouldn't have had any room on the tail to add on the stub stabs and the high aspect ratio fences that drop down from underneath the stab would have presented a tail strike problem)
It is interesting to note that the DC9 sends bleed air to heat the leading edge of the horizontal stab, yet the 737's tail is completely unprotected. I understand that the issue is not one of tail stall, it seems that the hot jetblast prevents ice from forming on the 73's horizontal stab.
regards,
8N