1. Spammer signs up with fake name (something like John Smith) and either has no profile picture or some picture pulled off Google. 2. Spammer adds users to their circles. 3. Users see the circle request, but, since this is just some random person adding them, they don't reciprocate.
In this interaction, spammers don't have an avenue of communication except via their username. If they could use a pseudonym, their username could be www.getcheapp1llz-ezi.ly, which would be as effective a way as ever to get people to go to their site.
On a site like twitter, where usernames are not limited by any criteria that I'm aware of, the spammer would have the medium of username as a communication method.
Now how much of this is relevant to modern times, where spammers are mainly working on SEO for spam websites? I don't know.
For what it's worth, I know a few people on G+ who clearly have a pseudonym as their name and they haven't been banned yet.