I guess it's the difference between: a) blogging your thoughts about an article and linking back to it, which encourages people to read the original so they know the context for what you've written.

VERSUS

b) quoting all or almost all of the interesting bits from the article, and providing a link to the original.

In the latter case, there is no reason or incentive for the user to visit the source. In the past couple of years I've begun to think this is the wrong way to go about things.