@Mo Jangda - Well, for any plugin that is hosted in the repository, if they don't have the links turned off by default with an option to turn them off and on, they are candidates for removal or suspension. So you shouldn't have to worry too much about those on the repository.

I agree, a powered by link is helpful. I'm routinely clicking on those links or looking for that text just to see what the original theme was. But I usually know where to look when I'm wondering about specific functionality on a WordPress site. As a last-ditch effort, I can always email them and ask them.