The geometry is different between the two types of bikes. Tri bikes typically have a steeper seat tube which forces the biker to a bit more forward lean. This design eases up some of the tension and degredation of the legs while saving more for the run.
The downside (as I see it) is that if you cannot get into the aero position with the aero-bars, you will have more weight on your arms and therefore fatigue your triceps more. A tri bike really is best set-up for the tri-bars.