Legion's multiplicity is due to his powers, and so is not really DID. DID is when multiple individual consciousnesses arise in a single brain of its own accord. It's something the brain does to itself, creating these other people from its own substance. It may borrow ideas for personality traits from observations of other people, but in the end it's still something the brain is doing to itself.
With that in mind, Legion's powers are something his body is doing to his brain, sucking entire other people into his own brain, along with their own mutant powers if they have them. Those other people in his brain are not created by his brain, they are literally other people who have been sucked into his brain by his powers. That is not DID. It bears a resemblance to DID, but it is not actually DID. What Legion has is, in fact, called "mind hive." http://tvtropes.org/pmwiki/pmwiki.php/Main/MindHive
Another example of Mind Hive: In some stories of mine, I have a race of aliens who copy the minds of their wisest ancestors into their own brains. They're multiples, yes, but they don't have DID because their condition was caused by an outside force, and those minds were copied from another body into that one. They are an example of Mind Hive.
This was cross-posted from http://fayanora.dreamwidth.org/1349216.html
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