While bitbucket have some similar features to github (or vice-versa), the features are not the same. So if you want to use only one interface, and you prefer github for projects with multiple collaborators, bitbucket isn't really much of an alternative. Or, if you prefer bitbucket, but is also professionally involved in a number of projects hosted on github, you're still stuck with two interfaces (this is likely the case for pretty much everyone, as almost everyone will have a dependency of some kind hosted on github, and at one point or other you'll probably want to/have to deal with upstream).

This would be true even if bitbucket was (subjectively) better: assuming one values having one consistent interface more than the "best" interface.

I don't necessarily think github's interface(s) are better than bitbucket (or that either are good, for that matter) -- but I can certainly relate to the desire for having a consistent interface, to lower cognitive overhead.

For me, that's the main argument for using Free/Open solutions, that one can self-host: one can guarantee consistency, which in turn can save time. There'll always be a balance between how much time is needed for managing such solutions, and between stability and stagnation.

All that said, it's hard to deny that github managed to leverage the network effect much more dramatically than either self-hosted CVS, stand-alone bugzilla+wiki or Source Forge managed to do. (The latter probably because they didn't realize what they business model should have been: not ads, but charging for forge-services. Then again, AFAIK github isn't profitable, either, yet?).