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True, but I never see SSH go down, unless there's a network issue, in which case I lose all my connections anyway.

Indeed; however, if you have a lingering control file (because an old ssh process was killed, or there was a power failure and it was somehow not removed), it will either refuse to use it, or (occasionally, and I haven't been able to pinpoint when), would just wait there forever.

edit: a typo

Sounds more like a bug than a disadvantage

Another annoying "feature" is that opening a new shell with ssh -X will not enable the forwarding if the original master connection was not enabled with it.

I have these shell aliases configured, to check and delete the master connections. (Often I end up with stale connections when I have a VPN up, and then just sleep my laptop.)

alias ssh-MasterConnection-check="ssh -O check " alias ssh-MasterConnection-exit="ssh -O exit "

+ then you use them as 'ssh-MasterConnection-check host' etc.