If there are branches with commits that aren't referenced elsewhere, i believe deleting the branch will eventually cause the commits to be "garbage collected". So you can delete abandoned experiments and keep a clean house. On the other hand, commits that are referenced elsewhere (perhaps merged into a main branch) will stay. But you could still delete the branch it was merged from, and avoid having the list of branches grow huge.
You can delete commits from Hg using 'strip' (enabled via the 'mq' extension). But this is strict deletion (as opposed to marking it for gc after a grace period) and you can't push an "unreference this commit", so you need admin access on the remote to strip a commit there. If you push branches to bitbucket for review, the abandoned commits stick around as anonymous heads until you strip them via the web interface, but they still stick around for anyone that pulled from you. The Hg community is working on a system [1] for dealing with this, but it's still not ready for general use and I find it significantly more complicated than Git's model.
