I've gotten most of my party in MM1 to level 6 now and also found a nice bunch of items to deck out my chars. I'm quite happy with the performance of my chars atm (especially the casters start to shine the higher their lvl it seems) but my robber is really a problem for me.

Armor wise he is the second weakest char due to the restriction to ring mail, whereas even the cleric can wear chain. Then he can't use 2h weapons and up to now I haven't found any real good 1h weapons (he uses a battle axe). Thing is he takes a lot of dmg and only does about 1-4 dmg in combat if he manages to hit at all (and I tried hard to boost his accuracy @ char creation). That leaves the ranged weapons, but again I've found lots of good bows but never something better than the regular crossbow you can buy at Sorpigal shop and the dmg output here is very low, too.

Have I mentioned he fails at disarming traps quite a lot? Well I would be interested in how you are using the robber class in game and how you integrate them into combat.

The Robber is only really useful for disarming traps and unlocking doors. That's actually really important, though, and he will get better at it as he gains levels. The best loot in the game is usually behind some really deadly traps, the kind that could wipe out the party if you can't disarm it, so you'll want a high-level Robber around.

In combat, Robbers aren't very good. I think I usually kept mine towards the back to use ranged attacks, but not all the way in the back because I wanted my spellcasters outside of melee range in order to use their best spells. Since the Robber has restrictions on the weapons he or she can use, you will want to find magical ones (i.e. any weapon with a "+" on it) for your Robber, as that helps with both chance to hit and damage. There's also a special crossbow for Robbers that you will hopefully find eventually, that will greatly improve your ranged attacks.

You could also give your Robber magical items. These can be activated to cast one of the spells in the game, and might be more useful than having your Robber attack.

If you move on to MM2, you might consider getting a ninja instead of a robber, because they're better at fighting. But I ended up importing my MM1 party into MM2 because I'd frown rather fond of them.

Robber is almost a must have class in MM2, also because of story reasons. In MM2 robbers get extra attacks at the same levels as ninjas and archers (but level faster), and can do extra damage with backstabs.

MM2 introduces the option to hire two extra party members if you want, so you could always hire a ninja in addition to your party. And as kmonster said, robbers are actually important in MM2. I'd forgotten that. Keep exploring! You'll soon find some good ways to earn XP quickly, most of which can be repeated. For me the toughest part was getting my sorcerer leveled up since he gains levels more slowly than other characters. Fortunately, I found ways to give extra XP to him, and once he got some of the more powerful spells the game got a lot easier.

Also, I think you'll find that your fighters gain an extra attack at level 8 which will make them much more potent in battle.