The issue, IMHO, has never been knockback; It's been the careless use of it. Unlike other blast secondary effects, which don't really affect enemy positioning in such a detrimental way, irresponsibly using KB can lead to aggroing other groups, ruining buffs or debuffs that rely on enemy targets, or otherwise complicating fights. If the KB user was solo, nobody would care, because they'd have to deal with the consequences of their own actions, but in a team environment, that can lead to team wipes or greatly extending the time it takes to complete an objective. It's somewhat similar to when someone immobilizes an enemy or group too early, or if someone uses a phasing power on them at an inopportune time. In short, when you're on a team, it's no longer about just what you want to do or how you want to use your powers; Work with the team or leave.
Note: If you are one of the players that DOES know how and when to use KB effectively, then I have nothing but admiration for you!