I can see where some authors could take offense at this, see GRRM.
But, for the most part it is complimentary and kind of endearing. The whole Star Wars and Star Trek extended universe have sprung up from a form of fanfic. I'm a big fan of the feedbooks versions of the parallel universe DC/Marvel comics that are out there. I have friends that write and read extensively in their favorite series. They prefer Supernatural and one used to play in Anne Rice's vampire worlds.
If done right and properly executed, some may add to the actual original work that it is derivative of. Should I become some big wig, I'd probably let a couple of my sandboxes out in the open under CC for derivative / non commercial. I think there are a couple of other guys here that feel the same way.
Or, you could take the Grantville Gazette way and have everything be approved by the original creators, which could be pretty cool...
I used to read and write some fanfic when I was much younger. It's kind of like the proving grounds of writing. I don't know if I'd want anyone to FF my main universe though, they'd probably do a better job than me.