Enmity bars are

a. relative - you can't see your overall raid dps, you can only "see" each member's. Also burst vs sustained looks different.
b. difficult to judge - subject to human error unlike raw numbers
c. limited in what information they convey - you can't pick up on errors in rotation and whatnot
d. restricted to the fight - you cannot assess the information afterwards and
e. detrimental to performance as attention is required during a battle to watch them.

Enmity bars are far inferior to parsers.