Scouts deserve special mention, for being both cheap and stealthy -- a combination which makes them good for holding things like supply items, extra gems for forays far from labs (say, when you want to ration gem usage), and for carrying things you _might_ want to use in battle like a Staff of Storms; perhaps you have your own archers and don't always want storms, but will be storming a castle with loads of towers soon or something like that.
It's also reasonable to have cheap commanders to ferry reinforcements, or to soak up Seeking Arrows
, or to have a few regular troops on 'bodyguard' so the local province defense commanders aren't totally unprotected when the cheap hordes all run forward.