As much as it pains me to say it, Pav is right this time.
When it comes to assassins having spells being part of an argument here, if you direct yourself to the prc information page, you have to have some training in casting arcane spells, and most of the time when someone is learning magic, even if it is with the intent of killing people certainly exp retreat would be something that would have been looked at by anyone with 8+ int to learn. See invis might be harder to justify in character, but I don't think its too far up there seeing as how assassins need to be able to kill anyone, and any wizard ever knows invisibility.
Now that my rp justification rant is over, lets talk mechanics.
As pav said, you need to be a level 5 assassin with 16 int to be able to cast a level 3 spell that has a dc of 18. That means the earliest you can cast it is level 10. If you were to use it on someone similarly leveled, Clerics, Paladins, Rangers, WM, RDD, blackguards, monks, dwarven defenders, divine champions, fighters and Barbarians all have 7 fortitude base and thats assuming no save feats (Which is rare) and no con mods, have about a 50% chance of not succeeding the save, and it only scales up as levels go up, poison with a lesser dc is an even lower chance. That means over half of the classes available on the server with 10 con (Which never happens for those classes) and 0 save feats (Which is even rarer) Can just pass the dc by existing.
At level 10, you have 2 attacks with your main hand, and two with your off hand assuming you took imp two weapon fighting (Why wouldn't you) so instead of spending an entire round trying to cast a spell, you could just pop out of invis and death attack with your 10 (Base assassin dc) + 5 (assassin levels) + 3 (Assumed 16 int mod) 18dc death attack, which you can get two off in a single flurry with four attacks a round, and they are flat footed so their ac is about their touch ac (Which is all the poisons have going for them.) Which gives you a higher chance to apply your status effect which is PARALYZE instead of minor str drain, and if you feel so inclined you can always apply a spider venom to your weapons while you are invis before you strike, meaning you can attempt to poison them while you try to paralyze them with about the same dc (If not more). The argument could be made well what if you fail, use it then. Except, if you poisoned your weapons, you are getting 4 chances of poisoning instead of one, while still doing damage, and you didn't invoke an attack of opportunity.
As far as rp applications of these spells when you want to humiliate someone. Those situations are so rare and far between dropping a hundred gold on a scroll and just sitting on it for a few years is perfectly viable. And the gold cost of exp retreat/see invis is almost negligable when people pay you 10k+ for a head. There are other arguments for adding lightstep or one with the land as a spell suggestion and there is no complaints, despite those costing 200+ a scroll and buffing stealth, one of the most complained about things in the game.
I don't see buffing the poisons ever being viable, unless the dc was pushed beyond what should be reasonable, they were made to be weapon applicable(So just attacking with another poison isn't strictly better) or they are a ranged touch attack so you could slow someone while they ran, honestly slow is a better spell than poison is at the moment it fills the same role, but better.
