I wouldn't use
Michael on a girl - but I think I understand the appeal - if you like the sound - don't have issues with boy's names on girl's - and perhaps are honouring someone.
The name isn't that different to
Michaela (incredibly popular at the moment)or
Michal (pronounced in the anglicised way - and which I love:) - and it is infinitely nicer than
Michelle (unless you're French and can pull off
Michelle with a beautiful French accent :).
I also know two people who have honoured their grandfathers - by giving their daughters male middle names (both
Charles). I think it's a variant of the current trend of using traditional names and honouring names, but giving it a slightly different twist (it wasn't a longstanding tradition in either of their families to honour grandfather's - they just loved the idea).
so - in my usual long waffly fashion - I think they may have been thinking any of the above when they gave their girl
Michael as a middle name - or they could (maybe) have just loved the name :)