In DA:O, taking a thief or a warrior means you are only 30% or so as strong as a Mage played and levelled up with the same efficiency.
In BG, making a non-dual/multi thief means you are, likewise, around 30% as capable as any other classes: you can 'make do' if you use other classes by using your companions, but a thief is plain useless in BG AND has low-life, meaning that the odds of you dying are much greater compared to any other classes (even mage, as you are useless as a thief).
In BG2, using a thief or a warrior is, likewise, nerfing yourself- unless you are playing a trapper.
Let's not even talk about the joke that's the Bard.