In appreciation of the testing done, I have to say Attritioners aren't BAD, they're just not powerful enough to risk asteroids on until you've built your siegecraft. In a the two games I've played with spirecraft, I usually bring them along with my attack fleets. They tend to help keep losses down.
Additionally, when used on heavy defense worlds, they help. The problem is they don't help enough. They just sit there, and are about as useful as a stack of Basic Is. That is, they might help kill a few weak ships. When you're getting Mk III/IV waves, the MK III/IV attritioner doesn't really matter anymore, they really only had enough fight in them to make a difference against MK I/II.
*shrugs* Then again, there's a reason I don't play with spirecraft on. I'm not enthused by the units enough to make it feel worthy of dealing with additional exos. A golem changes a battle. A spirecraft just... meh, helps. And martyrs are really the only unit I'd consider worthy of it but they're OP, and I get the equivalent sacraficing roughly the same price of core fleet to clear a wormhole.