Creating marketing materials first clarifies a bunch of important things around your business: who your audience is, what your model is, how are you going to reach them. It also gets you speaking your customer's language early. All good things.

I tend to go this way too. In place of a speculative pre-startup business plan (though it grows into that, naturally, I find), this has a lot of the same benefits.

The downside is that I've produced a lot of vapourware, and have been in the position that I've conditioned potential customers to be disappointed. It isn't always easy to see what is feasible, and this can easily lead to over-promising and under-delivering.

If you can walk that fine line, it is an okay approach, I think.