Personally, I tend to ignore them. I just don't see the point in RPing an NPC. If people want to write fanfiction and all of that, that's fine. As was mentioned earlier, I don't have to read fanfiction if I don't want to. If Val's hanging out in the Quicksand and Thancred or someone walks up behind him and thwaps him on the back of the head, he's forced to respond to that. I feel it's kind of egotistical to think that someone could write/assume everything about a character they never made up. There's just too many variables, many of which have already been listed here, that could come into play. Not only that, but while the RP may be personal headcanons, they're being written in a medium that everyone responds to things which are canon. If it was done somewhere else, privately, it wouldn't be such a big deal, and I don't mind people that do.
To be honest, I really just don't understand it. I write/RP because I enjoy making characters and throwing them at whatever the world has to offer them, often changing them in various, surprising ways. That's the fun in RP for me, you know? Looking back a few years after making a character and seeing where they've ended up, as well as the various events (some small, some large) that helped shape them on their path. I like seeing all of these new, unique characters and the uncertainty of what they are like and how they will react to things. When you see X canon character, you already know (at best) how they are going to act, or at worst, have to deal with someone that quite honestly plays them poorly.
To be honest, I really just don't understand it. I write/RP because I enjoy making characters and throwing them at whatever the world has to offer them, often changing them in various, surprising ways. That's the fun in RP for me, you know? Looking back a few years after making a character and seeing where they've ended up, as well as the various events (some small, some large) that helped shape them on their path. I like seeing all of these new, unique characters and the uncertainty of what they are like and how they will react to things. When you see X canon character, you already know (at best) how they are going to act, or at worst, have to deal with someone that quite honestly plays them poorly.