CSS resets are bad idea and shouldn't exist. Every time you see paragraphs without margins, or lists without bullets, it is probably a fault of CSS reset.
Browser default styles aren't really good enough to be usable IMO; yes they have paragraphs but with zero margin and tiny text they're not exactly readable. Twitter Bootstrap is pretty much a bug report against the browser default style. Given that current browser default styles are half-assed at best, I'm sympathetic to the idea that not having a default browser style at all would be better.
The whole point is to make everything look the same across every browser in existence. Also to remove collapsing margins.