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Pros don't use them - hence neither should mere mortals.
Velominati rule 60 if you're interested in that sort of thing - personally I'm not. http://www.velominati.com/the-rules/
In fairness, they don't do anything. My wife seemed to think I had to keep them on her bike 'because they keep the air in'.
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They don't perform any function other than ensuring the valve doesn't damage the tube/tub during packing and transport.
Once installed on your bike, when you're topping up tyres, you just let a blast of air out to clear the valve of dust/dirt or if you've got a puncture you're usually changing the whole thing anyway.
And valve caps and probably other non porn features but I'd ride the shit out of it.