As I said, Coraid uses it for SANs. The question you're asking is equivalent to "Windows can play games, why doesn't Linux have any games?" Some of the things you might do with Unix Plan 9 cannot do. However, the reverse is also true.

You seem to seem to be looking for "an alternative to Unix." If that is the case, Plan 9 probably isn't what you're looking for. It is, however, useful in ways that Unix is not. It probably won't replace your specific Unix needs, though it largely replaces mine. Hope that clears that up.