To me, the real power of open source is accessibility. If I had to buy a commercial app even to discover whether 3D is something I want to pursue seriously I wouldn’t reach first base. Even if I do ultimately move to a commercial app because it has features I need, Blender will be the cornerstone of my development as an artist, and I may never need to look elsewhere.
To clarify, as a would-be stock photographer I use Photoshop rather than the GIMP, which is what I learned my image editing on. The reason? Better support for 16-bit colour, integrated file browsing (Adobe Bridge) and metadata support, and a range of plugins such as Primatte Chromakey and Noise Ninja that significantly improve my workflow. These issues are probably not very significant for most people - the demands of stock photography requires them.
I teach some basic design to computer science students (part of a web design course) and I use the GIMP. I want them to practice at home, and don’t encourage piracy. Maybe some of them will pursue an interest in design.