If you have a million posts it's a lot more than a few bytes in the post table. And there would be one extra query only for people whose IP changes all the time. Searching duplicate IP Address would get much quicker. But I suppose when the parser searches for the postid usually it uses the postid index which does not get affected by IPaddress, so maybe you're right and the performance gain for regular day to day forum browsing won't be that great.
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