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There are also dangers to having comments on a user profile.
They will be by nature personal and involve emotions, which can easy lead to hate messages, flamewars, trolling, etc. Also the things written in comments, may affect the privacy of the subject.
You may have a look at some social modules, like yoghurt, where I think this may be possible.
I hope every user has at least moderation and acceptation rights to the comments of his account.