people might avoid bullying, but they can also avoid accountability for bullyingThis argument is nonsensical unless there is an actual, consequential penalty for violating the "real names" policy. (Which would, of course, be even more of a monumental disaster.)
Suppose you are a troll or a sockpuppet or a spammer and you want to screw around on a Google website. Well, the fact that you're not allowed to call yourself "LadyAda" or "Lady Gaga" or, for that matter, "mechanical_fish" is no problem. You can just call yourself "John Smith". Or "Steven P. Jobs" or "George W. Bush", for that matter. It's not hard to fake a real name. And there's no meaningful penalty for doing so, even if you get caught. Most likely Google will just delete your account. No big loss for a troll. New accounts are just a click away.
Now, if you are LadyAda or Lady Gaga or mechanical_fish, Google's policy is a problem, because you've spent a certain amount of time building up your pseudonym. It has a history and you're identified with it and you've bought the domain names and given the press interviews and written years worth of blogs and opened some businesses and built up a ten-million-strong international fan club who comes to all your concerts. Gaga doesn't want to sign up as "John Smith" or anything else, because it will literally cost her money not to sign up with her chosen name, "Lady Gaga".