Actually I think they are useful in lots of situations. Most of creatures you face don't have class levels, so they wouldn't have access to that spells. Even at high levels there there are still some creatures that aren't immune to them, even some chars, like aria pointed.
It's true anyway, that even with high DC poisons they're still resistable, but with max fortitude there's always 5% chance to take efect. So I've been looking for different ways to create/use poisons.
A good way to save your purchased (or hard to adquire) poisons is by using Body outside body (Wu jen, 7th). This spell makes up to 4 copies (suposing you cap your level at 20). Each copy has a copy of your mundane equipment. Poisons are way too expensive, but they are still mundane. So having a lot of poisoned daggers with... let's say... Black lotus extract... then acelerate them, and if you have the quickdraw feat (or the weapons have the aptitude), it's like 9-12 (12 if you have one level of full BAB an +1 CL instead of a Wu Jen level) attacks CD 20 for 3d6 Con damage. At level 13 (the level you can use BOB) it look pretty well. And this is kinda basic strategy.
By the way, what is the spell you mentioned? (the level 3 that makes you immune to poison with long duration). I don't know it.
