Using a Reposition when the enemy attacks you can't really do a whole lot except maybe cancel a flank. When you use it in response to someone attacking an ally at level 4+, it can totally cancel the attack in progress if you reposition him out of reach of your ally.
Beyond that, take a look at how sweet Greater Reposition is--free AoOs for your whole team. Pretty awesome, and ultimately, more useful than the Trip (Trip is only valuable at low levels).
Greater Trip only gives an AoO to allies that are threatening it right now. With Reposition, you can drag them around so that your allies don't have to be so perfectly positioned to get the AoOs.
And yes, enemies are "immune" to trip earlier than they are to Reposition, but honestly, all Combat Maneuvers become worthless late in the game except maybe Grapple and Dirty Trick. They just become worthless at different rates.
I play a flowing monk. Reposition is very useful in certain cases.
Repositioning an enemy so you and an ally can flank. Repositioning an enemy so they are no longer backed against a wall. Repositioning an enemy so they can no longer block the door you're trying to fight your way through. Repositioning so they are no longer blocking the hallway (A flowing monk can keep 5 ft adjust forward, reposition behind them, let ally's finish off, and keep moving defenders like this until they run out of repositions).
Another use is positioning enemies where your allies can use their special abilities. For example, if someone has charge and the ability to knock people back, you can reposition someone next to a ledge, then ally knocks them off.
Again, it's not something you can use every time, but it does come in handy. It's one of those abilities that's of limited use except when it's useful and then it's really really useful.
Oh, and it's also easier to reposition than trip some creatures, like wolves, or things that are immune to trip like snakes.
What's plan B for being useful when combat maneuvers aren't working?
Smartass answer: don't be a monk.