I live in the Italian countryside and my connection, though "permanent" in theory, totally goes black-out now and then, and the disservice can last up to a few hours. This is frustrating as it is (you feel "powerless" when you can't even check your mail or a wiki page, or the weather forecast): it'd be insanely worse if I couldn't play a game I bought exactly to kill time when I'm home with time to kill, possibly with a faulty connection.
Even if the rate of faults went down in the future, there's no way I'll ever buy a game that requires online status. No way in hell. Also, unlikely as it seems, Blizzard may go bankrupt tomorrow and all the servers may go down. What happens to my right to play the game 20 years from now, assuming I'm still around? I do play games from 15+ years ago right now, so the scenario is not an absurd one.
Companies that go down this road will never win me as a customer. I may be "just one" lost customer, but I am an ASSUREDLY lost customer.