For site owners who want to go further than this, there are extras you can add:
This kind of thing is useful as a means to AvoidLegalRisk: at a stroke, the owner is rendered practically immune to any kind of copyright LegalThreat. It also swings the balance of power between users and admins heavily in favour of the admins. This ConcentrationOfPower is probably something to be avoided - on meatball we instead advise leaders to DevolvePower.
More charitably, it allows a BenevolentDictator to license some or all of the site under some form of OpenContent license in the future, without having to get buy-in from every single contributor, past and present. This is appealing, given incompatibility issues between various CopyLeft licenses, and the impossibility of predicting future legal and social changes.
Such terms are something of a PricklyHedge to potential contributors, especially those who you might most want to attract. Promising not to abuse the license can, for trustworthy god-kings, reduce the costs, but it's still not good.
If contributors assign copyright or copyright compliance, they should be aware that the person or group they're assigning copyright to may not always be as friendly or as flexible as they might be themselves. Cf [StallmanVsFiddes]
The above text is PrimarilyPublicDomain