If you know when encounters are going to happen (when you go into the dungeon) and you know there are only going to be a few encounters, and you can stop having encounters any time you like, then persistant is not very useful.
However, if that is the case, the DM isn't really challenging you.
The point, Pax, is that when you need to protect your entire party from *, you can. If that thing isn't a threat any more, then, the next day, you can use the slot for something else. Think of it as a magic item with infinite trade-in capacity. Instead of spending your money on basic magical gear, you can persistant a bunch of spells and buy more exotic items
As for every one's favorite, dispel, I don't like it. If the only way I can ever challenge a party is to open with a fleet of dispells, there is something wrong. Most monsters and basic classes do not have dispel. Not every group in the game should have access to this. But to hear you talk, they should. Persistant only exaserbates the dispel problem.
The detect spells are persistantable: detect evil-60'range, duration for concentration. Incidently, concentration is a standard action, so concentrating on the Blessed Aim for 24 hours would be silly. Curse of the brute can give a combat based cleric an amazing boost. Who needs int and cha? You beat things down.
Pax, you have never seen what persistant can do in the hands of powergamers. Perhaps you should look at it more carefully.