Aren't the Black Spiders still 100 gold each? 10 Spider riders (ie an investment of 300 gold) can take most indy provinces, but can 3 Black Spiders do that too? The usual upkeep advantage sacreds have is not relevant with spiders (riderless spiders are free from upkeep), so just the basic investment matters. I've been under the impresion one needs 5 or 6 black spiders to do the job of 10 brown ones, but I haven't tested this out in a loooong time.
I think I have to try this out
If 3 Black spiders, even with a bless (build a temple in a province, and build indy priests to cast Bless ad nauseautum, and you can still recruit mages from the fort(s)), can act as an expansion party, then that has to be done for the shere coolness factor. Black Spiders *are* cool, there is no question about that (I think), but just being cool has not been good enough for the steep price (except to eat elephants and giants, but those are not common enough usually).