1) Security. You're going to have to come up with something really fancy (read expensive) so keep the homeowners and any of their guests/kids from tampering with it. Also keep it a secret, $Xk of gear would be a good target for thieves.
2) Reliability. Even a halfway competent datacentre will have very high reliable power and networking. Some guy's house? I'd wager less so.
3) Like everyone else said, warm seasons.
So, if you need to host something that doesn't require any security and you're happy with poor uptime, it's could be an option...