Can you give an example.. I cant seem to figure it out...
Ranger or Warrior with 2 weapon combat but without the pre-requisite skills that can't equip 2 weapons;
light wizard merc with 2 levels in heal mastery (a Curion pointed out, seen the same merc)
and so on.
When searching for higher tiers mercs I have often found otherwise interesting NPC with one (or even more) useless skill.
Seeing how higher tier mercs have a higher wage and cost more to resurrect it is a bit strange to find that they have unusable skills.
I have found useful to pick up tier one mercs with 1 useful skill at high level and upgrade them to higher tiers so that they can use better armors and weapons.
Warriors and rangers don't get access to the better armor and shields, but when you pay a 21 level NPC 5 gold and not 80+ and the ressurrection cost some teens of gold pieces it is a good advantage.
It become easier to add another low cost merc to make the group more powerful and leave the buffing skills to the main character than to search for a merc with the right skill set up.
I would suggest than when you increase the tier of a NPC, the wage and the cost of resurrecting him are set to to the lowest cost for a merc of that tier.
At the same time the player get to chose 1 skill for the merc that get raised once every 3 levels. That way a tier one merc upgraded 2 times up to tier 3 and that has fought with the player long enough to get to level 20 will get the pay of a level 20 merc and 3 useful skills, like the best mecs you can find in the tavern.