Well, my thief is equally worthless in melee or ranged combat then.
There are a few thief-specific items that can help with that a little, but yeah, overall they're a bit worthless in fights.
I've never tried playing the game without one... it might work OK for some things, but I think you'd probably miss out on some of the best items. The traps on the high-quality chests are pretty nasty and can sometimes take out the whole party.
In MM2 and beyond, you can take a Ninja instead of a Robber... they're thieving skills are not quite as good but they're decent fighters. Of course, most people prefer to import their MM1 characters into MM2, but you could always swap out your robber for a ninja in MM2 if you were so inclined (while keeping the rest of your original party).