Its my understanding the UCI rules require the tip of your saddle to be 5cm behind the center of the BB even on TT bikes (triathlons don't have this rule).
That's where my saddle is anyway, so what benefit would a TT frame give me (other than the aerobars)?
__________________
“Courage is going from failure to failure without losing enthusiasm." (Churchill)
"I am a courageous cyclist." (SpongeDad)