It is all goal related. There is really no reason for a bodybuilder to arch when benching. However, if you want the biggest bench you could possibly have, then learn how to properly arch. The key word is properly, a correct arch is NOT Joe Blow the gym rat with his ass a mile off the bench putting up 275 for a couple of reps with his spotter molesting the bar screaming "it's all you", lol.
It blows my mind that people take their feet off the ground to bench also. It is a compound, multi-joint movement. There is no possible way to only use your pecs to perform the lift. As far as power output, I like the firing the cannon on a tight rope example, and I will just leave it at that.