I think Sky just has a rather old system that can't easily be changed, so that's the way it will be for some time. I am sure Sky will think 'if it aint broke...'.
By its very nature, it's an online service so I think if Sky had set up its service today, it would copy the others and allow multiple installations but only 1 (or 2) of them to work at any given time. Easy to check.
The offline stuff is already set to expire pretty quickly, and even quicker once you've started watching, but it's not the end of the world. Like Spotify, you could restrict how many devices could store offline content (at the downloading stage when you'd have to be connected).
To be honest, if I'm watching Sky Go then I'm probably out. If I'm at home with my big screen TV, then I'll be using my Sky box.