I don't think a developer, photographer or a writer needs a band new Mac Pro every three years.
Writers don't. They don't need Mac Pro at all.
But some photographers do. And a lot of developers do as well. Like me for example. Developing 3D games, from simple mobile games, to some way more demanding is possible even on MBP. But not great.
And VR is coming, as is VR gaming. And if it takes off (and it probably will), some developers will need to upgrade even faster then video creators.
And anyone who makes money out of their computer is a pro. And that means writers as well. It doesn't matter if they are journalists, bloggers, or fricking GRR Martin (and before fans kick in, I know what he uses to write GoT)