Limiting those with non-mainstream needs or desires (like pseudonymous interaction) to niche sites, effectively amounts to total segregation. Those people may want to interact with their real-life and online-only friends using the same system (e.g. Facebook or Google+), and some of them want to use their nicknames or handles to do so. If your friends would be put off by "darklajid" or "nitrogen" commenting on your posts, then you can prevent the posts for your real-name friends from being visible to your pseudonymous friends (if you have any).
I'll take my argument further: Google+ and Facebook grown beyond mere commercial offerings to become essential services like e-mail or the telephone system. Forcing certain classes of users onto a different network prevents them from interacting with the main body of the network. I consider this an inherently bad thing. Imagine if certain people were told they could only use niche telephone networks that were incapable of dialing the network that everybody else uses.
I'll take my argument further: Google+ and Facebook grown beyond mere commercial offerings to become essential services like e-mail or the telephone system. Forcing certain classes of users onto a different network prevents them from interacting with the main body of the network. I consider this an inherently bad thing. Imagine if certain people were told they could only use niche telephone networks that were incapable of dialing the network that everybody else uses.