There's a slight difference between a git push and a full mirror operation. A git push only synchronises the current branch. Even git push --all doesn't necessarily push all reachable commits - tags, other named refs (eg. stash), the state of the reflog...

It's somewhat useful as poor man's backup, since forgetting to push to the other remote would be a pretty human thing to do. I guess you could do some kind of bash alias to push both remotes, but that would also assume that both are available from your network location (not sure if Git can have n remotes with one push alias).