Mage + Thief = Bard... Wrong, since went a Bard can open a look and sneak attack?
Fighter + Thief = Assassin... Since when it can open a lock, or have you seen a Assassin circling its prey?
I believe having multi-class could bring more colorful characters around, allowing to have new abilities to our characters and new combat tactics. Having one class with no chance to learn new tricks is abnormal, you gain experience and levels, you learn to be stronger in something but you are too dumb to learn anything new even if you have 20 Int to learn to cast or 20 dex to learn to picklock.
As a D&D player I can tell you there is something to do with multi-classing that would bring a whole new and fresh wind to LoS.
We can achieve multi-classing with hardcore RP, like Nrel did, he was a Cleric and turned into a Dark-Knight, keeping his abilities of cleric and gaining some new Dark-Knight abilities.