The problem is that, while there are a a bajillion thief NPCs in the game to recruit, there's only a handful of Clerics. Only two of which are available at the start of the game. So you're not usually going to be in a position where you've run out of thieves and need to start converting clerics
I like to leave Branwen as she is and think she is better than Viconia,despite the fact i'm usually evil,I've just taken Sharteel and dualled her to a thief,first time i've done it and she's now a thief with true grit.
Comments
Best bang for your buck is Cleric 7/Thief 8.