Actually, you'd be surprised how much effect pushback has. I think the thign with pushback is that it is linked to another relatively unique concept in Warhammer 3 - no turn limit.
Without a turn limit, the whole pressure that you need to maximise value from your units falls away, meaning that you can spend a lot more time manouevring - 'wasting turns' in later editions. But, when you have unlimited turns, suddenly the notion that you push a unit back 4 times in a row can make an 8" difference in a line - with no sign of that combat ending. UInless the rest of your line has done the same, you'll have to ask yourself a very real question around the 5th follow up - can I afford to be 10" away from my line?
Also, a real fundamental to the pushback is that it gives the victor the choice of continiung the fight or not - that's not available in later editions - you fight until someone breaks. Pushback disconnects the two units, allowing either to reform, change direction and so on. Perhaps they have different equipment they want to use - Imperial crossbowmen might find it better to push a tough unit back, not follow up and then fire at them with their higher strength crossbows, before receiving the charge again, for instance.
Finally - it gives you some options against units like the undead that don't rout. In later editions of the game, these units are referred to as 'tarpits', because once you've engaged, you're pretty much stuck. By being able to beat them off, you get the change to strategically withdraw.