BitBucket is owned by Atlassian for 2 years now and they don't look like they're about to go out of business ($60mil revenue in 2011, 400employees, worldwide offices). When they bought it, it was them who set private repos to unlimited (when @jespern was running it alone, it was set to 5), so I believe they know what they're doing and that it fits their business model. Over 5 collaborators need a paid plan and that's just how they set up the pricing scheme.
As far as I can see as an active user and lurking through their blog and bugtracker, BB is actively developped, so I assume you don't have to worry about them running out of business, looks like it's a valuable asset in Atlassian's software portfolio (at least valuable enough to work on it).
Yeah, and people payed GitHub to host their private repos --how did that go?
It's $6/month to take care of my most important assets. That doesn't even buy a meal at McDonalds anymore; it's worth it.