Some users might want to have multiple AskUbuntu accounts. There are a number of legitimate reasons for wanting this. I won't try to list them all but here are a few:
Separate accounts for work-related and non-work-related questions/answers (to make bosses and supervisors more OK with the user spending time on AU during work, when asking work-related questions or providing assistance with work-related products).
Separate accounts because one account is effectively shared with another person.
Separate accounts because the user needs support with something that could lead to social or political/governmental backlash if some more information were revealed...and they have revealed that information with their first identity.
Separate accounts because a question (or information) provided in the first account could exacerbate IT security threats if taken together with information provided in a question asked with the second identity.
Separate accounts because the first account personally identifies the user, and the user prefers to remain anonymous on the Internet when possible.
Separate accounts to protect the identity of other people, such as relatives. In particular, if a user has an AskUbuntu account associated with other StackExchange sites, there could be information revealed on those sites which, in combination with some questions about Ubuntu, could potentially violate the privacy of other people. One possible example of this could be a parent who has revealed enough information about his/her child on Parenting.SE that the child's teachers and peers (though not the public) could identify them, then asking here on AU about a child with a certain disability using Ubuntu.
Do we allow this? Or should multiple accounts belonging to the same user always be merged, no matter what?
Of course there are illegitimate reasons for having multiple accounts too, like sockpuppetry (and the related avoidance of disclosure of relevant commercial affiliations), spamming, trolling, and bullying. I'm not suggesting we should permit using multiple accounts for those purposes.