I'm been convinced, especially since that story of the abandoned bantam/darkhorse invasion storyline came to light, that del rey wanted to tell a Star Wars story that could only be told in the medium of novels. I've only read a couple books from the series, but what I did read really gave me a vibe of "this could never be done as a movie." You don't even see comics set during that era.
Although I will admit that after hearing about what happens with Zonama Sekot in the Force Heretic arc, I understood even further what their whole point with the series was. It gave it a kind of fantasy feel, which of course is what Star Wars itself has.
Another thing is, well, y'know how a lot of us prefer to think of the original trilogy as the real story and the prequels as just prequels? I think this kind of backs that up because it shows how grand Luke Skywalker's life ended up being. Born at the end of the Clone Wars, fought in the Galactic Civil War, helped build the New Republic, fought people from another galaxy far far away and proved himself the greatest Jedi ever.
Troy Denning's post-NJO trilogy also sounded cool because of the whole "now, let's get back to where we left off before the Vong showed up" thing. Using that concept of the Killiks also had me interested.
As for Legacy of the Force, acknowledging the old Marvel stories was a pretty random move, but like the Killiks in Denning's trilogy it's cool that they're mining these old and ambiguous materials for ideas.