Vague goals beget vague methods...

I don't believe so. I think the entire point of doing a PhD is to gain the ability to think independently and analyze loosely defined problems. Unfortunately today many PhDs are exactly what you say: delving into something academic to the point of beating it up to death. This leads them to only think about their own problems and forget about the scope of it within their academic domain. I think that researchers should in fact focus more on what is going on in their academic region of interest.

Isn't the entire purpose of PhD to struggle with a topic until you understand it so deeply that you become the world's expert on that tiny piece of academia? Will this lead to lazy uninformed researchers?

yes, the purpose of phd is to sit around listlessly while pondering vague thoughts on someone else's dime.