have one potentially good use which their description hints at -- especially on a low crystal map. 10 gold may be less valuable than the 4 crystal it'd take to magic arrow something to death, or the two-three turns or whatev' it'd take to magic spark a corpse into existence. Hurl a militiaman at something and you've probably pretty promptly got a corpse you can turn into something undead
They also approach decent en-masse thrown against a single target to drain its stamina while something ranged plinks away. I've held off some warriors and suchlike doing that.
Militiamen also have something interesting. If you can manage to get a few levels into them (good luck, but I've had it happen) they can upgrade into spearmen. I was a lil'surprised when that happened.
I actually just had some pretty good luck with a barbarian/shaman party lead by a scout -- I was using necromancy pretty heavily on my main hero, but didn't have the crystal income for much of the fight to keep re-raising zombies and didn't want the moral penalty on my sword/pikemen, so I went neutral/unscrupulous to fill in gaps in his battleline. Barbs are actually pretty tanky and hit decently hard, plus have that little ranged attack -- they're about in the same position as zombies, really, just faster and much harder to get high leveled ones (Mage with 5 necro and 3 summon was spitting out 13th level zombies, iirc, which are basically doombeasts.). Shaman are... surprisingly useful, especially one or two supporting some ranged power (e.g. a scout). The free curses are quite nice for breaking down something's armor a bit or cutting into something's offense, and the magic ranged attacks are non-trivial in a lot of situations. I'll likely start throwing in one or two for any future commander led bowmen parties I make. All in all, they're very mobile and easy to move around, which can be tactically more useful than the pike/sword/crossbowmen, sometimes.
I wouldn't touch the brigands or the thieves with a ten foot pole, though. Thief poison might be useful, sometimes, but the fact that they reduce province income is just... no.
Ah, and Re: Scout vs. mage sniping... from what I've seen, scouts can snipe way, way
better, at least earlier on. Find an alright bow and go heavy into the ranged stuff, and 20-30+ damage per arrow before mid-teens seems pretty standard, which... magi have a bit more trouble pulling off, at least for me, so far. Mages are more useful in general, though, by a long shot. Just expensive to make use of, heh.