I *think* that they say a matress cover helps prevent SIDS as there may be dust/allergins in the mattress which affect the babies airway (this is something that they believe can contribute to SIDS).

(Ok, I just read the site and this is what they are saying)

However I think... well doesn't EVERYONES mattress get like that? Not everyone has a plastic sheet cover, and not all babies die from SIDS... so it can't be as simple as popping on a plastic sheet to prevent SIDS. There are obviously a number of things that contribute, I mean the theory is there and everything, and I guess its worth a try as long as it wasn't dangerous, but I definaetly wouldn't use it and not do other SIDS prevention like sleep safe, cot positioning, etc.