We already have a word that means "not interested". It's "uninterested".
If you use disinterested to mean not interested then you are depriving the language of a separate word with a separate meaning.
If you are disinterested you are unbiased, you have no personal interest in the matter.
A good police officer is disinterested but not uninterested.
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As is traditional for Pedants' Corner I fully expect to have made an error in my typing above. :)