It's been a while since I've even looked at the 12+, much less anything actually meaningful, but supposedly WMKK/Mike does reasonably well in the ratings. Not "great"...nowhere near it, in fact...but reasonably well.
The key, however, is that Mike is far, far, FAR cheaper to operate than a "normal" radio station. The staff costs are much less since there's no DJ's...just the sales guys, really; I wouldn't be surprised if most of the other operations of Mike were rolled into job descriptions for other stations at Entercom. Although admittedly I wouldn't be surprised if that's NOT true, either...but either way, the lack of DJ's & similar airstaff saves a few hundred thousand per year in salary & benefits costs; maybe even over a million. So even if the ratings aren't "tops", and thus the ad dollars aren't "tops", the overall return on investment is rather nice.
Personally, I think it's kind of the point that Mike not have a morning show; if you wanted one of those, you'd listen to another station. If you want MUSIC in the morning (which a smallish but no doubt sizable audience does) then you listen to Mike.