I've encountered two people that I knew more as aquaitances online that I found out later made deliberate attemps to use multiple identites. Under normal circumstances there's nothing wrong with wanting to hide who you are or have different information at different sites to make it harder to steal your info', but these were different circumstances.
The first was probably the worst -- different e-mails, different names, different ages, even a different home state. While certainly not illegal in of itself, however what he was doing it was obvious he was trying to trick people into doing things that wouldn't do for him had they really know who he was, so it certainly wasn't moral. Just because you can and just because it's not illegal, doesn't make it right, after all.
The second was very, very similar. But he was doing something that wasn't legal However it appears he made all these changes not to fly under the radar of the law, but for a similar reason above: to trick people (not for an unlawful or necessarily devious reason).
I go by three handle names online, one of which is simply my real name. If one looked hard enough at various sites, you'd probably find half a dozen different birthdays for me. I use at least a dozen password combos and variations and just plain unique ones, which are not things like "love", "puppy" and "1234".
It's okay to hide who you are, but if you are trying to purposefully decieve somebody to defraud them or commit some nafarious act upon them, it's wrong. And if you do it to create more than one account someplace or get around their system for some reason, it's clearly not okay,
Some sites are so invasive or are becoming so imvasive, I purposefully give false information to try -- as best as reasonably possible -- to keep some annonymity. My original e-mail back from 1994, has become mainly now a dumping ground for registrations so the spam goes there (though over twenty years of not replying to the spam and almost never posting it publically for a bot to find, has resulted in little spam anymore).