As to the reasons for the press to attack decision on console, it's largely focused around targeting. We experimented with the "feel" of auto attack and discovered that it didn't actually offer you as much control and freedom as pushing to attack. Let me see if I can explain.
On the PC, you have both a currently selected target, and a "free floating target indicator" in the form of your mouse cursor. You can be attacking one guy while simultaneously lining up a new target by moving your mouse overtop of him. As such, even with basic attacks it's possible for you to quickly spin and hit new targets in the midst of the melee because you have a precision tool for doing just that. By my estimation, this is a good thing.
On the console, you have two "known" ways to select your target: camera control, and D-pad. Those are unchanged. However, in testing, we found that the overall feel of combat could be improved by adding a third, and far more natural way to select target, movement stick deflection. Thus, if you are in the midst of a melee and see an enemy breaking off to attack your mage, you can simply deflect the left stick towards that target and the target will change when you issue your next attack.
And since your next attack is on the next button press, you can immediately change targets mid-fight. So your rogue can break free of a scrum to cut off the foe attacking your mage, and so on. A warrior can spread attacks between two foes to build aggro and so on. The overall feel is very responsive, and largely made possible by push to attack, which allows us to achieve similar results to the free floating target indicator of the mouse, but to do so in a way that, as SirOccam pointed out, "feels very natural" on the consoles.
Obviously, the concern is "oh noes! My targets will change if I sneeze!" This isn't really the case, however, as there's a LOT of code involved in keeping your current target sticky and generally feeling right.
Also, since someone asked and I didn't see an answer when I started posting this, "A" on the 360 is equivalent to "X" on the PS3.
