DS2 with the expansion pack made multi-classing worth it. Multi-classing in and of itself isn't worth it within Dungeon Siege's class framework. However due to the ability/skill system introduced to complement it in DS2, the expansion pack was able to take that and create new skill sets for two multiclass options (combat mage/archer, and nature mage/fighter).
I'm not at all a fan of the system of improve-by-use, though. Perhaps something like Fallout's skill system would be best here, where you decide each level where to invest your skill points, and you can invest them in anything (no classes).
But we already know that DS3 contains classes, and not the fun Diablo type classes, so... poo.