If i read it right, the Gen travels through the Elemental planes seeking the spells the Sha'ir wants, speaking to the many Genie & Djinn and native inhabitants if they can grant the spells to the Gen, which is why you need a diplomacy roll for each of the spells you ask. The spell is then brought to you, so you can promptly memorize it, although you forget about it in Sha'ir Level/hours. Which kind of sucks if you don't know exactly when you'll need offensive spells, but is really useful for when you are in a dungeon crawl and need that *one* specific spell. In a way, it combines the versatility of the Wizard with the ready availability of the Sorcerer, but has that drawback that it's practically IMPOSSIBLE to request spells mid-battle, as they're delivered several rounds later, by which time the battle will most likely be over. There are also several disadvantages. While one can argue that a Wizard's spellbook is just as vulnerable, the Sha'ir Gen, in a way, is the Sha'ir spellbook. Being a living thing, it is subject to slaying, and the killing of it carries a hefty XP penalty (200xp per Shai'r level). Not only that, you don't even need to kill it, you merely need to strip away it's ability to Plane Shift. Most paranoid high level Wizards ward their lairs against teleportation and plane shifting... It's one of the most common defenses in dungeons( Sha'ir couldn't, for instance, function properly in the Tomb of Horrors, since Plane Shift has big chance of causing TPK's there). At each combat, it would be a major liability to have the Gen out, but also a big drawback if you're running out of spells, or if the spells you do have aren't suitable for the situation. All in all, i'm really torn with the Sha'ir, i don't see it bringing that much to the table in comparison to the core Wizard/Sorcerer or Cleric.
But i digress. Regardless, i am not so sure about the Elemental planes in Ravenloft, but i guess, if they're open, then it's OK.
EDIT: Ninja'd while typing. That is a very good point link, that would be a good adaptation, didn't think of it. So, just maybe it can be done, up to the DM i think.