The Sidhe Lord is a great early game choice, or late game when you have multiple smaller forces in addition to your main force/s. Basically he has just enough magic to cast Wind Guide so he's ideally suited to leading a cadre of longbows, whether that be early conquest of indies, patrolling in late game, or as a territory-grabbing auxiliary force.
The Cu Sidhe definitely seem to me to be the extra troop to recruit when you max out your resources. If you have lots of income and a high dominion then you have the possibility to make quite a few Cu Sidhe each turn, in addition to the Daoine Sidhe. I happen to think they're very useful too...
