I think that Tom is correct that writers whose primary goal in writing is to tell an entertaining story should be judged more on those merits than on others, in general. Sanderson can be a lot of fun to read (except in Elantris, where the writing is downright terrible), and I enjoy his books. However, it's sometimes discouraging that writers who have prose/style weaknesses are encouraged not to work at improving that aspect of their stories, because their fans cry out on every forum that it's the entertainment that matters, not the writing.

Whether that's true or not, I feel writers who have prose/style weaknesses would create better books if they put effort into improving that; and writers who have plot/character weaknesses would benefit from improving that. Having a weakness doesn't make someone a bad writer -- every writer has a weakness -- but if they were encouraged to acknowledge that without worrying that doing so would hurt their sales, while at the same time seeking to improve their weaknesses over time, the end result could only be better books for the readers.