dtgreene: If you plan on getting Imoen, you can use Yoshimo until then, and you won't be left without a thief. (Imoen rejoins right after Yoshimo leaves.)
If you are speedrunning, Yoshimo is good because of his backstab (which can kill the SoA final boss in one hit if he shapechanges into an Iron Golem). (Note, however, that in this game, speedrunning is very different than casual gameplay.)
jsidhu762: I know you said that speedrunning is different but how would Yoshimo make it to the final boss (I`m assuming you`re referring to Irenicus) ?
First, Yoshimo is given permanent shapechange ability when fighting the Shadow Thieves. This is done by having him use Limited Wish via item swap or having him talk to the Limited Wish Genie. To make it permanent, Yoshimo needs to die before it wears off.
Second, in Spellhold, there is a portal that leads to the Underdark. But, of course, aren't there locked doors (that can't be picked or forced) in the way? Well, there's a solution to that. Have Yoshimo polymorph teleport through those gates. Fortunately, only one character needs to be by the portal to teleport the entire party through.
Polymorph teleporting is an engine exploit that takes advantage of a quirk of shapeshifting abilities. If you shapeshift from a small form into a large one and you do not fit, the game places in the nearest open space where you *do* fit. If the nearest spot happens to be on the other side of the wall, you will end up on the other side of the wall. Another use for polymorph teleporting is to avoid the permanent penalties involved in one of the hell challenges; instead of opening the doors, polymorph teleport through. Note that summons can sometimes be useful for filling space when doing this trick.
If you're interested in this, why don't you watch a speedrun? You will see how it differs from regular gameplay. For example, in the first level of Irenicus's dungeon, one character polymorph teleports to get the key while another goes to the portal to use the key. There is a lot of multitasking involved as well as tricks like dismissing a character and having the character rejoin on a different map as well as using Limited Wish to break cutscenes.