Finding out a project is on Bitbucket or uses Mercurial has actually stopped me wanting to contribute. The additional hassle of remembering how Mercurial works, digging out my BitBucket credentials etc, is a pain in the backside.
You can use bitbucket just fine to host git projects.
I use a mixture of bitbucket and github for different projects, not wanting all my eggs in one basket, really. I find them pretty much equal in many respects - except being able to have private repos on bitbucket is very convenient, and github pages is also very useful. I wish bitbucket had something similar.
