That's very true, though if you only go to places you know and trust then Groupon isn't doing it's job for the restaurant! They're simply giving a discount to loyal customers.
I guess that's me though, everyone is different. I have been stung too many times in the past with bad meals and I am probably the fussiest customer you will find. If I had been going to a place for years and they started serving bad meals, I wouldn't think twice about not going there again. I don't generally complain about a meal, not going there again is good enough for me.
I like the insight into the Groupon experience as a whole though, this is probably what would happen to me so I'm glad I had never taken the plunge.