The geometry from a TT frame is completely different. Typically you'll see a much steeper Seat Tube angle, and I believe the Top Tube is a little shorter. Also, they're often made out of aerodynamic tubing to help minimize your drag coefficient.

They are not comfortable for long rides. If you're looking to do a lot of TT'ing, then perhaps it's not a bad idea, but if you want to buy a TT frame b/c it looks cool, I think you should reconsider.