I'm sympathetic to the economic feedback problem you're describing. I think the BSL addresses the concern about undercutting. Sure, people could pirate your software, but short of that, it probably makes more sense to implement from scratch than either wait ten years, or fork code that is ten years old, which is all your competitors could do with the source. Honestly, I would probably just pay for your service after spending a few hours spot checking the source.

Anyway, I'd love to hear your thoughts on the licensing model, even if you're not considering it at spideroak.