I suppose the tradeoff here is when you get two non-Gmail users, johndoe and john.doe, who are legitimately different people, one of whom gets very confused and

I suppose the tradeoff here is when you get two non-Gmail users, johndoe and john.doe, who are legitimately different people, one of whom gets very confused and is actually unable to use their correct address. On the one hand, that's a (presumably extremely) rare corner case - on the other hand, some applications must handle those corner cases.