Poor analogy. Its more akin to having a cheap guitar you got for free and someone suggesting you go spend a lot of money on a top brand one to learn with. It makes more sense to learn the transferable skills on the free package and then if you like what you do and reach the limits of the software, consider paying more at that point. Additionally, most photoshop users don't use the vast majority of features in the software, myself included. For working on digital output, you can do everything you want in GIMP anyway, photoshop's real feature advantages come into play for print output. The rest is really down to which you are most used to (productivity), and it's better to develop a taste for something thats free if you can, than for something that is very expensive, especially when they both do the job you want.