First, I'd argue that the accepted fact that they are the industry standard is, in and of itself, a good justification when you're talking about buying a computer for use primarily with one app (or a related suite)... your product is more likely to conform to standards that those in the industry will expect.
Second, since they are the industry standard but are clearly not the cheapest way to go, one would have to assume that other companies have done the cost/benefit justification themselves and have arrived at the conclusion that Macs were, overall, a better purchase.
Third, even outside the industry, the Macintosh - both before and especially now with OS X - appeals to the artistic and graphics professional (and amateur) due to the way that the OS works with you, merging the use of all your applications and the operating system functions, as opposed to Windows, which merely, at best, doesn't get in your way as you work in a given app.
I use Windows systems every day at work, and I use Macs every day at home. I'm a software engineer, so there's a pronounced PC bias in the software tools available, and I still find the Mac easier to use (and, for some reason, I feel more inspired, creative, and imaginative while using Macs). I believe that productivity on a Mac in a field where they are the standard could only be even better (vs using a PC) than it is for me.