Don't expect everyone on dance floor to stop, so you can get focus. About the groom and bride walking the aisle? About bride throwing bouquet? You think the M focus is fast enough for those moments?Once upon a time, I managed to get moving atheletes in focus with a Minolta X-700. Now, if you mean you wouldn't shoot a wedding with an EOS M because you need fast AF achieve good focus, that's sounds like a personal problem, to me.
Not relevant. I am surprised that you are trying to defend using EOS M to shoot wedding
. Yes it can get some great to decent photos, but you will surely miss many. As Jim O pointed out the manual focus cameras of past had nice workarounds. EOS M is probably one of the worst camera I have used to capture the moment instantly (and its not only the AF, dead time between the shots has a good part to play too).
Regardless, this discussion seems to be pointless:
Can EOS M be used to shoot weddings: Yes
Can you get some great wedding photos: Yes
(Imagine a crazy scenario with 10 assistants carrying M toting nice L lens; randomly you would get some great keeper for every moment)
Are there better options than using M (provided same/similar glass): Almost everything else (except P&S and iphone)
Lets move on to something more meaningful....