[Azaghal]

You can get Jaheira back one time. She'll wait where faceman said. When you dismiss her for a second time she'll be gone forever IIRC.
Anomen is a good substitute as a plain cleric, especially after (ahem, very slight spoiler)
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certain changes he will undergo during the game. He's a pretty good melee fighter.
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End of spoiler!

Aerie is not so bad, you can never have enough mage power. But she is rather weak and has little HP.

Viconia is fun to have in your party (and she's sexy), but Anomen is the better choice as a cleric. Best thing about Viconia besides her obvious advantages: natural MR (25% IIRC).