If you develop B&M as patiently as you develop your personal character, then you will have a fighting force up to whatever standard you set for yourself.
If B&M are really as useless as many peple say, I find myself wondering why they don't just cut them loose permanently and fill their slots with something presumably more useful. Even Marnids' paltry inventory slots do not justify maintaining his presence if he is as incompetent as some claim.
I presently have a Borcha who [which?] for a few points difference mirrors my own character and performs with about the same effect [mounted archer]. The Marnid in this crew is heavy cav/maces - basically a prisoner generator. The trick - for me - is to find out how much to give [and take] from these two until you get the performance you expect. The AI isn't perfect, but it is consistent. Sometimes less weapon capability, sometimes no shield, these are the things that it takes to keep these guys operating in the parameters that you set. I rarely see either of them, and they rarely screw up. This is a case where a lot of micromanagement IS required and does return on investment.