Its more a versatility thing, sure its a nice bonus to have more tools, but as iwe and other stated... having five fireball spells, doesnt really make you better... and then its also needing several stats, and you can have a billion buff spells, but you can still only concentrate on one... and then there is the action economy...
Also, while heavy armour is tempting, in order to use it you need to place points in strength or you will more or less allways be encumbered... even medium armours can get heavy with 8 strength...
So yes casters get nice stuff when they multiclass, but non-casters is where you can make the really stronk builds, if you know how to min max the system... but again, that is my take on it... IF you got more spell slots, yes then it be far to goo, but its alittle of a trap, couse if you cast a heal, its one less fireball or counterspell... its better to have a a few casters, as that gives you more buff spells...
There is one thing im curious to check out and that is sorcerers twinned spell, as that is the only way in PnP you can cast two concentration spells with one cast... not sure how Larian wil handle that
