@Parlance I have to agree with everything you said.
There are way too many drawbacks for playing as a melee instead of a caster, especially for a rogue-oriented character. Not to mention playing as a caster also means each character gets an additional summoned unit to fight in combat for you, you don't have to spend turns walking to get to your enemies and you save turns from melee opponents trying to reach you which is huge.
Also a caster can be much more versatile because intelligence boosts all schools of magics, so you only need to invest into one attribute, meanwhile if you want to go hybrid with your melee/ranger character you need to spend points into intelligence leaving you weaker in your main area.
I think Larian devs had too much fun with the spells and the enviromental combinations that they forgot about balancing with melee. I find that the only reason someone would go for melee is to artificially inrease difficulty. So much for diversity.