It also helps control people who just want to sign up and start flaming answers.
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I would bet that this is Mahalo's way of ensuring that only people who are at least somewhat regular contributors to the site are affecting other members' ratings.
I think it's a little stupid also (I'm still not a Yellow Belt, but I'm almost there!), but I can see the potential for abuse by non-regular-contributors. I would not be happy if someone continually ranked my answers as unhelpful just because, for example, they didn't like my avatar or something.
Regular contributors, I would think, are more likely to take their answers (and other answers) seriously.
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quality control, in a nutshell.
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In order to weed out the people who are not serious in helping people/only want to screw around (ie. nominate the worst answer as a joke). On top of that, this would prevent people from abusing the system by creating a multitude of accounts and voting their own responses as best. This, in turn, leads to a site that actually gives good answers (unlike some other sites out there).
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This simple method of user control ensures that users cannot sign up for multiple accounts very easily to downgrade others' answers. It also stops novice users from negatively rating others's answers until they further understand the site and the general quality of questions and answers provided.
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