I don't use published settings, with one exception*. I don't want to deal with the "Canon Nazis" saying "That never happened" or "Its supposed to be like this" or "So-and-so would never do that".
And it can lead to a point where the PCs actions directly contradict something in canon. If canon says "Starbase X existed in year Y", but the PCs blow it up before that, you end up having to ignore the canon. If your going to ignore the canon anyway, why not just make your own setting, and have everything the way you want it?
*The exception being Forgotten Realms. Sometimes, I'll set a D&D campaign in the Realms.
And it can lead to a point where the PCs actions directly contradict something in canon. If canon says "Starbase X existed in year Y", but the PCs blow it up before that, you end up having to ignore the canon. If your going to ignore the canon anyway, why not just make your own setting, and have everything the way you want it?
*The exception being Forgotten Realms. Sometimes, I'll set a D&D campaign in the Realms.