Even thou it's not the subject your right. Which is why i don't see why people work so hard on there site and not have there own domain.
I can potentially see one reason for someone not buying their own domain. If all they want to do it create a mini-site of just one page then they could accomplish this with something like Squidoo. Since Squidoo does not use a subdomain per page/site (like blogger etc) webmasters can avoid the sandbox as the domain is already highly trusted. Apart from this I would always buy a domain of my own.