I do have a 1-user SQL Toolbelt, and I've registered my copy of Multi Script.
I see your point about multiple instances on one server. I think the real value of this is being able to run scripts across multiple servers.
Frankly, I don't see this one going anywhere.
Sure, you'll sell a copy with each toolbelt, but paying (even) more for additional servers for what amounts to a enhanced Query Analyzer/Management Studio tool isn't going to endear this to too many folks.
On the other hand, if you would have released this without that restriction, this could have become another classic.