While this is valid approach, there are inherent dangers with it that have to be taken into account.

In my past life, I was a copywriter and most writers in those days would start on Print Ads exactly in the same fashion. They would go about and write the copy and the art director would put the visual around it. So there is a reason this comes naturally to people - this is one of the ways people have been always worked.

I however would always do both visuals and copy (I ended up doing both on most of my work). I believe the advantages of this approach are are:

1. It makes the communication a lot more coherence to the idea and makes a better presentation as well.
2. You are able to make the tweaks much more easily as you are playing with both copy and visuals.
3. You end up being a lot more creative