Hypothetical question: Having pre-paid the fee to register a domain for n years, do you really own the domain or does the registrar? What happens when you use the same company as both registrar and host and you default on your hosting payments. Does the company have the legal right to take your domains in lieu of fees?
I dont think a reputable registrar would combine owed hosting over a paid for domain. In other words, they would kill your hosting but leave your domain active. Thats the reputable well known ones though.
The only people that "own" domains are the actual extension registries (verisign for .com and .net). What domainers have are contract rights to use a domain. If people bothered to actually read their contracts when they register a domain they will see that their rights are very limited in reality.