I would say that the artist is the more important than the software choice. There are many fine repaints, profiles, and other artworks which have been created with PSP and Gimp. I am now using Gimp for my graphics work. I make very little use of filters outside of the occasional default 'blur' variety.
There are a surprising number of amateurs (some rank) who will mention using the latest Photoshop versions. I am puzzled at this, since purchasing CS3 would cost more than my entire computer. Whether these people are laying out the credit card or stealing the software, I cannot say.
But either way, Photoshop has an incredible market share. This explains why you see so many support materials. Commercial graphics will very likely use Photoshop in some stage of their production, and the more refined plugins which are avaliable for sale often are targeted at the commercial workflow.