Nothing wrong with it, it’s just more involved.
At the very least, I'd have to write a full outline and a couple sample chapters, with no guarantee of ever getting paid for it.
I guess another thing would be with tie-ins, there are already certain parameters. You have established characters and settings to work with. You don't have to start from scratch.
Now there's a great deal of freedom working on your own fiction. It sounds like Christopher, for instance, likes that freedom with his original works. But I'd imagine there's a lot more work involved with the writing as well. I mean, you'd have to create everything yourself. And hope that your readers are intrigued and entertained by that. With Star Trek fiction, for instance, you already know you have a built in audience.
But it's a good goal to have, to someday write something original. I'm sure there'd be a great sense of personal satisfaction involved with having something say 'created by Greg Cox' or 'created by J.J. Miller'. Having something you can do anything you want to with.