Kev,
You've seen mine , it not a whole cover just covers the top half.
I thought they were well daft when some guys camped next to me with them in the US. But they do keep prying eyes away and the rain off tank bags and seats overnight. I was convinced enough to get one later on in my trip.
When touring with Louise we sometimes leave loads of stuff on the bike, tank bag, clothing ( all locked on in some way) and then throw the cover over the bike to stop people messing.
We did a Scotland trip a few yrs back and it rained for most of a couple of days , whenever we got off the bike to do some sightseeing we didnt have to carry around wet stuff and we didnt have to try to stuff wet jackets inside topbox/panniers alongside dry stuff.
You have seen mine on but it packs down into a pouch about the size of an A5 envelope only bulkier . I have the larger size ( although it still doesnt cover the whole bike, certainly not panniers but it covers the top case and everything down to foot peg level) but you can get a smaller one that act a bit like the old tonneau cover on british sports cars. Eitther one can be slipped on the bike in seconds without worrying about hot pipes or cans.
People take the piss but stuff 'em , I find it useful
I am going to the US in less than 3 weeks, let me know if you want one and I'll bring it back
Ian