With the higher MP counts, improved DR and pro-influenced features present in mid-to-high end APS-C cameras, is there a place for these traditionally enthusiast bodies in the professionals' bag? If so, what types of photography are they ideal for, and what types are they not?
No - too big and heavy. m4/3 is the way to go!
Seriously, I switched from Canon FF to Olympus m43 a couple of years ago. I've not had a single complaint, lost clients or any other nastiness since doing do. (I shoot primarily for magazines).
While there are some areas where the pixel count matters - e.g. high end fashion, more often than not 16mp is plenty ... it wasn't too many years ago that full frame cameras were producing 12mp and that was considered very high res.
What is far more important is choosing the camera the lets you get the shots you want. For some, a built in crop factor and lighter weight will result in more "keepers".