[Taqwus]

I can see at least two major reasons for giving gifts to an AI.

The first is to help the recipient against a third enemy which you're not immediately willing or able to confront directly. Perhaps the third AI is significantly more powerful than your recipient and is on the other side so your armies can't reach yet. Perhaps you're playing with an impossible Ermor and don't want to see it gobble up AIs that are otherwise not terribly good at combating Ermor. That sort of thing. These gifts can sometimes be chosen so as to minimize the potential impact when you eventually get around to backstabbing; e.g. if you're largely poison-immune, or simply going to avoid melee combat, then the snake bladder stick won't hurt you; likewise, if you've got no undead, it isn't a huge problem to hand out herald lances unless you were really counting on routing the enemy through Terror and such.

The second reason is far more exploitative and cheesy, and that's to send double-edged gifts which can seriously hurt the recipient if used unwisely. Examples would include Lycantropos amulets (cursed, berserk, eventually turns recipient into werewolf), extra eyes of aiming (one's fine, two's a problem), bane venom charms (disease spreader), rime hauberk (if he's not cold immune, probably will kill those standing next to him), bone armor (same bit, with life draining) etc. It's cheesy in that this is taking advantage of the AI's relatively careless use of items and that only the most careless or inexperienced human player would be likely to fall for it.