Alson, just something I've realised while playing the Wizard Slayer > Thief dual. It starts off decently, and gets better when your MR% starts picking up.
Then you dual to a thief and things go tits up
Your MR% vanishes, you are restricted to using magical weapons and armour and you have the pleasure of being a L1 thief. Great.
The main downside with this is that you can't use potions of invisibility, magical rings, necklaces etc until your thief level exceeds your Wizard Slayer level.
I went for the L24/L28 dual, which means I have a long way to go before I can become all powerful. I dream of hitting L25 as a thief, I will get use any item plus my beloved MR% back.
Since dualling at the end of Saughin city, I have advanced to a L11 thief, and have only just entered Ust Natha. I have not started or completed any drow quests (though I did kill the mind flayers and beholders).
Just be warned, it takes a lot of patience to make this dual work.