One good reason I like to stick with a basic canon is what happens when other version start tampering with it. It loses value.
Like Star Wars, they've gone all over the place.
In the the movie, the Emperor was killed. That was the defining moment, the tyranny was finally over.
But no, later in the comics, he was brought back as a clone.
Luke Skywalker stood as the one who would never turn to the Dark Side. Later, after the movie, in another story he falls to the dark side and serves the resurrected emperor.
Luke was supposed to be the last hope--no--in another story Darth Vader trained another potential Dark Sider years before he first encountered Luke.
Sometimes extended stories and universes tends to mess with a classic good story line.
Trek would be a weird mess if we allowed everything in the book world and elsewhere into it.