Yes I agree with you there, but I already have a Wildcard, so actually it's not an issue for me, but indeed... for people who want to save money it's great.
I'm thinking about removing the whole admin section from the normal install and move that part to the subdomain.
Now I have to figure out if, I think that's the most wise, I copy my whole WPMU install first to the subdomain how I ripp out the blog function on that subdomain. so that people can't visit their site too like :
blog.subdomain.domain.tld.
It's a fancy setup, but can be nice when you want to seperate the admin server from the real hosting server.