Since you ask what is better: 10% MR or 5% MR and +1 to ST, I think the former is better: you don't need any saving throw at all if the spell is blocked by your MR. Moreover, there're many damage spells in the game and while a successful ST would let you get less damage, your MR would let you get no damage at all.
You really don't need saving throw bonuses right now, your death and spell saves give you automatic save against most anything that allows a save. But magic resistance is always good to have. With Vecna you will have better magic resistance:%10 magic resistance is a huge upgrade over a mere %5, your chance of completely ignoring a spell just doubles with Vecna.
With +1 to saves you will have %5 better chance of a succesful save vs breath weapon (which is required very rarely, when a dragon breathes on you should already be immune to the element via protection scrolls/items/spells)
The community's views on this subject are not united, though. While some people think the Use Any Item ability should do exactly what it says, including the character-specific items, other people think differently.
Mods aren't really an option for me. I dislike the rule, and I think it weakens one of the Thief's saving graces.
The way I play, I'm totally cool with Imoen being the only thief in BG 1 and 2 (and I'll dual her at level 4 or 5 in bg 1). When you weaken the best Thief HLA, it makes the decision even easier.
Comments
Since you ask what is better: 10% MR or 5% MR and +1 to ST, I think the former is better: you don't need any saving throw at all if the spell is blocked by your MR. Moreover, there're many damage spells in the game and while a successful ST would let you get less damage, your MR would let you get no damage at all.
With +1 to saves you will have %5 better chance of a succesful save vs breath weapon (which is required very rarely, when a dragon breathes on you should already be immune to the element via protection scrolls/items/spells)
This is an intentional behavior, check http://forum.baldursgate.com/discussion/comment/409893/#Comment_409893
The community's views on this subject are not united, though. While some people think the Use Any Item ability should do exactly what it says, including the character-specific items, other people think differently.
"Why not?" in this case becomes rather a rhetorical question, really. You can use a mod: http://forum.baldursgate.com/discussion/26240/use-any-npc-item-add-on-for-use-any-item-hla
The way I play, I'm totally cool with Imoen being the only thief in BG 1 and 2 (and I'll dual her at level 4 or 5 in bg 1). When you weaken the best Thief HLA, it makes the decision even easier.