@faraday Counterpoint.
I gave this some thought a while back. I used to think the same thing that you just posted. What changed my mind about MU being writing is the following...
First, for a hobby about writing there sure is a lot of NOT writing. PBs have replaced character descriptions to the point where people have to race to pick up their favorite PB before it's taken. Some people are still into writing it all out and creating creative setting pieces on dug rooms, but it's not across the board. I'm sure there are other places (I'm just waking up), but there tends to be a lot of "move it along" and waiving of written description in lieu of pictures on a wiki.
Secondly, I think people speak of it as "collaborative writing", which i suppose it could be between specific players. As a whole? Its 20+ players all stabbing at similar plots with individual efforts and little coordination. I cant remember ever playing on a game where the plot outline was detailed beforehand and group conversations were had about necessary scenes, plot development, and who would be in charge of writing from X character's perspectives. I would say more so that players make a character and try to coordinate, but see @Pandora above; the end result isn't really intentionally collaborative writing as it might be described as "loosely collaborative storytelling environment where some characters are so clearly idealized extensions of the player that it can be difficult to do anything outside of what others want to happen". There is no chapter craft, no coordinated plot progression, and you could say that scenes are crafted, but most of the time (in Mu) a scene isnt something you go into with a point, but more to "see what might happen and it keeps going until someone gets bored or has to sleep". Turns are not taken in any form of novelization. A novel where every other paragraph is some writer taking a turn without pre-planning the content of said turn with the other author would be horrible. It reads differently.
So, I cant feel that this is evidence of actual writing, but a roleplaying hobby that considers it to be because they're emulating the style of writing. In truth, most published authors in the hobby have either left or consider their writing to be a separate entity from their RP hobby.
What happens in MU is something nebulous, but I would describe it more as a "text-based Second Life" before I describe it as being in the same vein as what Stephen King, Brandon Sanderson, or Jim Butcher (former MUer) do for a living.
In fact, the MU paragraph/turn style may hinder one's ability to get out of that mindset to make better, less purple, more impactful writing pieces. To be a writer, a MUer should get out of that MU mindset and dig into exactly what these novelists are doing, how it's done, and understand the importance of how those novels are put together. The writing style of an author/novelist is entirely different. There are no "poses", 8 paragraph purple prose entries are simply not novel quality no matter how much people think they are amazing, and the writing level of most contemporary writers targets the imagination of an adult at the reading comprehension level of an 8th grader.
So, counterpoint. I still agree with Mercedes Lackey. People can disagree with me as much as they want, but until someone publishes a 75000 word chapter of a 25000 word per scene log list of a MU that has 450 scenes, I'm gonna have to agree to disagree.