IF the retailers realy honour their warranties, why not just buy 2 cheap PSU:s at the same time? It's often cheaper to buy two cheap ones than one really good. When the first of the two gives up, the switch will only take an hour since you already have the replacement in hand. And it doesn't really matter if it takes a week or two to get the faulty PSU replaced.
it's a nice "idea" but keep in mind that it "can" do a blow back and knock out other parts of a pc when a PSU dies. Or even worse it can give sub par power/w to the machine, causing them to fail much faster...
thus why it's worth it to get a nice solid one IMO.