Get a life? Got one, one upfront!
My main reason for going to the top of any program is the "upgrade". Sure there are other benefits, but the upgrade far outwieghs any other.
This applies not only to airlines, but to hotel and car rentals too. I've recently made the switch to the Hilton products from Marirott because their diamond program is simply better then Marriott.
My loyalty is for sale, for sale to the highest bidder, with the best amenities and service!