Overcome git svn caveats

I am reading the CAVEATS of git-svn. via this SO question.

What I understand is that:

If you are mirroring svn trunk, branches etc on git repo, don't merge or rebase anything locally on those branches. Specifically master points to the trunk.

My point is that, this would make git-svn crippled. What's the point of git, if I can't branch from master and then merge that changed branch back into the master? Perhaps I am missing some caveat in the caveat that says it is ok to merge certain branches that have the same parent without any rebases.

Can anyone point to me exactly what needs to be avoided and what is allowed. I'd want the local branches, if necessary, those need to be mirrored and merged and most importantly git should do the merges than the svn.

If I am using git branches to mirror svn branches and I cannot merge git branches, whats really the point of git-svn anyway?