When I used WinXP I did not have .Net framework installed at all. Only once did I come across a piece of software that needed it.
Since moving to Win7 .Net 3.5.1. is there by default. I've not bothered to uninstall it / turn it off, but at the same time I've no idea whether I really need it (e.g. whether I have any software that uses it). Consequently I get loads of updates for it - they just go with the territory (but I've blocked the install of .Net 4.5.1. with no adverse effects - my understanding is that if something needs it it will ask for it).