Why choose a traditional frame over an aerodynamically streamlined frame? I'm not talking specifically about triathlon frames, just frames which have aerodynamically shaped tubes. As I understand it, both types of frame are legal for road-racing under UCI rules.

So, why would one choose a traditional frame with round or "squoval" tubes over an aero frame? For example, why would one choose to ride a Cervélo R5 with (more) traditionally shaped tubes over a Cervélo S5 which would seem to be more aerodynamically sound?

Does it boil down to personal preference, or are there specific conditions in which one out-performs the other?
Aero frames are generally slightly heavier than the standard frame you'd compare it to, though the difference might not even be enough for the average person to notice. Someone who regularly competes in time trials might want the increased speed on relatively flat ground, competitors will certainly notice the difference.

If you're not competing in time trials or triathlons and you don't have an extra couple thousand dollars burning a hole in your pocket, you would probably choose the standard, more compatible, traditional, and lighter frame that is going to perform perfectly well on your average terrain. If you frequently ride on hilly terrain, the added cost and weight of the aero frame isn't likely to be worth the trade off.