Oh, sorry, I think I haven’t understood you. You talk of the fact to select only one role like the system that I described, but with a merc with multiples ratios of proficiency ?
I don’t think it’s a good idea, when I select an engineer, I want to be able to do the best work of engineer.
That was a problem with Brink for me, to have a unique and persistent character. So you are a full soldier, and your team need an engineer, but you can’t switch engineer because your skills for this are too low.
If you have a special merc with 50%(defuse)/20%(hack), he’s good for neither of these objectives.
Finally, I’m not sure that the proficiency system could be great here:
Ok you take the role which permits to defuse a bomb but you are medic, so you are very bad to do this objective. This will result with what SplashDamage wants to avoid = Push the players to play with other mercs that they want because, as you have the role to defuse the bombs, you feel compelled to switch to engineer class.