[Baalz]

Yes, their sacred infantry are not bad, but they are capital only and not really good enough to carry a strategy in small numbers - as you say you mass them far too slowly. You can certainly mix them in as a bit of a bonus but I haven't found they make that much of a difference. For the cost of one of the Daoine Sidhe you can get three Firbolgs, and outside of the bless each Firbolg is almost as good.