i think the box is connected to the phone line so they can check what pay per view stuff you've ordered in the past month eg box office movies. it also lets you use the full range of interactive facilities.
re the dish - it will have to be in line of sight of the satellite. thats what determines whether it has to go on the front, side or back of your property. the sky deal includes "standard installation" ie in line of sight of the satellite and in the easiest postition for the installer to get at. it may be that if you want it placed more discretely, eg on a chimney stack, it will cost a bit more.
we have the full sky package and periodically i cancel bits on the basis that its expensive. we always go back up to the full package tho so i guess its worth it! i'm defintely going to get sky+ at some point soon.
they've never been nasty about me fiddling around with our subscription package.