Good for you! There's nothing I can't stand more than people who say "run this or YOU LOSE!" and that only 10% of a codex should even be considered. Nothing is so useless that it should be completely crossed out of its respective codex. Granted, some things don't have MANY uses, and those uses don't always end up actually occurring in a game.... but everything has some use somewhere... except for Dark Eldar.
The trick is to exploit the advantages of whatever you choose to stick with as much as you can. For example, for your Tzeentch Princes, one of the best psychic powers they can take is Gift of Chaos.
I play very Undivided/Mixed God lists, and I've actually recently stopping running my princes as Lash Princes, since my friends are now usually counting on it. After a little experimenting, I've pretty much converted to using Tzeentch princes with Gift + something else (usually Bolt of Change, sometimes Warptime) even if I'm NOT going for Tzeentch theme.
Why Gift? Well, it's usable in CC, doesn't count as shooting so you don't have to assault who you use it on, it's frickin hilarious to make Eldar players cry when you insta-gib their Farseers without their precious inv save rerolls (or even a Hive Tyrant if you roll a 6!)... oh, and because Mark of Tzeentch lets you do it TWICE!
On a non-Tzeentch Prince, Gift isn't nearly reliable or destructive enough to take as your only psychic power IMO. It's devastating in the right situation, but only works 1/3 of the time against two of the most common armies played (Space Marines and Orks,) has a short range, and has to be used before you move. By itself it's kind of lame... but with Mark of Tzeentch it becomes a LOT better. Being able to take both it and another, more reliable power makes it a lot more forgiving to take that "what the hell" shot at turning a Warboss into a Chaos Spawn, not to mention being able to do it twice while locked in close combat makes that 5+ roll against SM and Ork armies twice as likely to happen.