This sums it up perfectly. JUCO's are older, by definition, so it's almost like getting a upperclassman without having to wait. Wide receiver is one position where this can really pay off. You may regularly use 3-5 wide receivers in a spread formation. It's pretty tough to keep that stocked with 5 guys that can reliably catch the ball, even in some good programs.
The fact that you can so fully learn about a player's traits makes this less of a risk. If he's already a mid-70's in catch rating, that's a guy that could play immediately, or maybe after one year of redshirt. In that situation, it doesn't matter that you don't get all 4 years of his eligibility, because you don't need to "wait" for him to develop critical skills.