As a vid viewer, I can say that I stream vids for two reasons. The first is that it makes it simpler for me to watch it quickly as I can tab it open or watch right in the screen, and thus feedback the vidder. When I download I may end up forgetting about it for a while (read: hours to days) and by the time I view it's a chore to go back and find the original vid post.

Alternately, I prefer to stream than download because I sometimes don't end up watching the whole vid so being able to quickly see the first 30 seconds is more useful to me.

Of course, streaming or downloading speed is not a big issue for me, so I imagine experiences will differ.