It really depends on your usage patterns. I watch YT mostly for audio in the videos. Either it's music or some documentary or interviews or shows like This Week in Startups. Video is really not important and I don't want to waste my bandwidth on it. I watch all videos at 360p by default and only increase it manually when I really need to see the detail.

I also know a lot of people who just start a playlist with music and leave YT in the background the whole day. They don't even have that tab active in the browser.