bigfatdave, none of my guns have magazine disconnects. I don't see the point for me and agree that I don't see a purpose for them on concealed carry guns. When I was in law enforcement our issued gun did not have a magazine disconnect either.
However, to address a few of the points raised (both by you and others, and I understand that you were referring mainly to non-police situations):
How about avoiding the struggle, or not deploying a firearm if you are planning a wrestling match?
"...cops who have had their weapons taken..." Operator failure, a lack of training and/or a bad holster.
If you have time to drop the mag, you have time to pull the trigger.
I did a lot of hand to hand training with a police gun belt on. No matter how aware you are of your surroundings, you really don't have eyes in the back of your head. The "breaking up the party" scene described above is a good one. Lots of people around.
Our duty holster had a retention loop that you rocked down and forward to release. If someone tried to grab my gun, which was common in training, I could reach across with my left hand and block the retaining strap from traveling forward while delivering elbows with my right arm. This maneuver kept the gun safely in the holster, where the trigger was guarded, though all of the wrestling and rolling around.
While the gun was holstered, I could have easily dropped the magazine. It would have done me little good to do so, because our guns didn't have a magazine disconnect, but the point is that there are times for a cop when, no matter what you train and how aware you are, someone might get a hand on your gun without you having access to the trigger. If you had a magazine disconnect, it could be useful in that situation.
Keep in mind that these training scenarios were often 3 or 4 on 1, full contact with guys in red man suits. At some point most students had their gun taken away, usually after several minutes (it doesn't sound long, but feels like an eternity) of fighting. If a cop saw that he was outnumbered and that he was likely to lose his weapon (4 guys pulling your wrists away from your holster will usually get the job done) he could be saved by a magazine disconnect. Planing on emptying your magazine via the trigger isn't always an option.
All that said, I'm glad our didn't have them, and I don't personally want them on my guns, but they can serve a valid purpose for law enforcement.