A lot of people have pre-existing pseudonyms that _are_ their online identities. Should they not be allowed to use that identity in Google+?
Let me ask you this: should frivolous whims of minority dictate product decisions at Google? Or your own startup, should you have one?
The serious pro-anonymity arguments revolve around people who have to keep their identity secret or something bad will happen to them (they will be stalked, harassed etc.).
That is very different from saying "I want to be called spud because that's what I want and that's my Live Journal handle". A different standard applies to evaluating this vs. evaluating "anonymity to remain safe" arguments.
Insistance on using real names is Google's product decision. It's also an important product decision which will have a profound effect on how G+ will play out long term. Maintaining the culture of a large community is pretty much the biggest problem in that space and insisting on using real names (and not, say, adolescent or whimsical nicknames) is an important part of steering the culture towards civility.
Why do you feel that random people on the internet should dictate product decision to Google team?
Would you feel the same if you were running an on-line community, doing your best to shape it in a way you feel is good, and then random people on the internet demanded that you change your product in a way that they think is good but you don't?
I take issue with your premise. Anonymity is a scale including "real names" -> pseudonymity -> anonymity.
There are folks who want anonymity and folks who want "real names". Problem here is the "real names" supporters want to ban pseudonyms as well. I take real issue with that.
Pseudonyms aren't a recent thing. Stage names, noms de plume and masked balls have been around for millennia. The drive to purge that from our online public spaces is a real threat to society as a whole.