Never.
"Read my fanfic!" pretty much tells me what level a "writer" is at. It hurts to even use the term "writer" when it comes to fanfic creation.
The fanfic I see are:
1. I want to wank to the characters in X, so I'm going to write bad **** about them!
2. I can't come up with a universe.. or characters.. but I think I can come up with a story if I steal someone else's universe/chars!
3. I wanted the episode/movie to go this way, so I'm going to write that!
4. I worship this so much, I need to write about it, so I can add myself as a character! I'm going to be so cool!
5. I'm going to combine 1 and 4!
Generally just about all of it is cringe worthy/sickening.
You just want to cry when you find they have been sending their "ideas" to the creator for some time... or even better when the fanfic writer says the creator should no longer be allowed to write about those characters because the fanfic writer's stuff is so much better.
If you are fresh to writing and cutting your teeth on fanfic as it allows you to work on just a part of writing, that is fine but you probably do not want to show your stuff to to many people and you most certainly want to get out of fanfic ASAP.
Honestly, I respect the **** writers the most when it comes to fanfic.
This guy is totally wrong and I can prove it.
When you go to the movies this summer an awful lot of the offerings you will have to choose between are FANFIC. Did you love the latest Avengers movie but didn't care for teh comics? Well then the fanfic is better than the original.
We like to stereotype fanfic as "I'm writing this furry anal-vore Star Trek story with me as the Marty Stu main character, it is going to be awesome!" but fanfic is really just one person taking another person's characters and/or universe and deciding to set their story with it.
While some reboots like The Thing are so unrecognizable when compared to the original, most reboots are not just recycled plagiarism they are also fanfic.
Game of Thrones is now Game of Fanfic, sure it has suffered for it but we still get more episodes even if we now have to spend a lot of time yelling at the TV and facepalming.
Does Joss Whedon want to be called a fanfic writer while he rolls around in his billions? I doubt he cares.. because BILLIONS.
So in conclusion: not only can fanfic surpass the source material it can lead to a high paying job that makes you famous.