Yes, but that's why I use an extension like NoScript. If it's too badly broken I enable domains 1 by 1 till I get enough content, sometimes websites just look odd or behave odd but still give you what you came for. It also helps to have FlashBlock as well, which will stop plenty of flash adverts from loading in the background.
Maybe, but the typical person doesn't browse the entire web, they browse a tiny part of it. Could be the GP visits sites (such as this one) that don't all depend on JS and there's a positive feedback loop for them.
I've permanently whitelisted about 150 domains, and temporarily whitelist domains as I need them. It's not very problematic and pays dividends in general web browsing speed.