I think there is a general preference for maps that are not only good informational guides but also have high art value too. In this respect the programs that make maps alone usually are tailored to getting the informational bit in quick. The high art side needs a good skill in art and cannot be substituted through some kind of app wizardry. Sometimes all the 'Old School' maps which are limited to two color or very limited color are perfectly adequate for running an RPG game but rarely do they generate the amount of oohhs and Aaahs required to get onto the choice pages.

You should make a map to do the job its designed to do. Some people here actually don't like the high art and find it off putting to the job of specifying what is where.

I have a gray name but I don't make my living off of maps but I do write an app so I am a bit of a pro fraud. But most of the gray names might sell some maps on the side but very few indeed make a living off of making maps exclusively. For those who live off that income I don't think most of them do use PS. For the ones I can think of, they use real ink or GIS type apps. For the most part I think the ones doing most of the on the side map income are using the apps that they use for whatever main job they do.

Saying all of that I still think PS and Illustrator are the most powerful paint / vector packages around. But of all the abilities of them I suspect that all of the features that are used here to make the maps Gimp and Inkscape has too. Like Torstan says I expect its just in memory management and stability that is what drives people to use the big gun big price tag apps.