I’ve spent some time trying to think up a suitable rejoinder to your post. But in the end all I could think of were these twin notable quotes by Horace Mann:
“Every addition to true knowledge is an addition to human power.”
“Be ashamed to die until you have won some victory for humanity.”
But in those quotes, perhaps, is one answer to your question. (Never mind that I'm not all that keen on the idea of working solely to improve the lot of humanity as such. The general principles embodied in the quotes are sound enough.) Sure, a person might wish to adopt an alter ego to trick others, to play games for self-amusement, or in order to serve as his or her own echo chamber. Then again, a person might see an alias a useful tool for conveying lessons that can’t otherwise be taught. Point is, I expect there can be reasons for creating multiple nicks that don’t relate to vulgar mischievousness or low self-esteem.
On the whole, though, I agree with you. “Guess who I am” charades usually get pretty tiresome pretty quickly. And honestly, it’s all well and good if someone wants to reinvent his or her online persona, but then in that case it’s only fair to expect that person to put some genuine effort into crafting a new and improved – or at least different – voice and perspective. Failing that, it seems to me that an erstwhile familiar poster might just as well identify himself or herself.
Cheers.