It's just that photo editing software has a relatively high learning curve. I think (anecdotal, I have no references for this) that due to the cost of photoshop many users start experimenting on pirated copies. Mostly kids & hobbyists. Years later they may buy a copy or take it with them to the workplace. By that stage, they're used to photoshop to the extent that it's the only option.
I'm sure there are some Open Source developers out there who do want every possible user, and want, more than anything, to oust their leading proprietary competitor. I'm only among them when I'm thinking like a business man, and trying to figure out how to make money from my software.
Which anecdotally, isn't the case for the lead devel, since he started yesterday as the lead devel for a new startup around the corner from Directed Edge. :-)
I've been an Open Source developer for over 10 years...and while it's nice to have a lot of users (particularly if you want to make money on the software in some way), the kind of users you have is more important than the quantity. I'd take one good Open Source fanatic over a dozen Photoshop pirates any day. The pirates don't do anything and they have little respect for the developers of the software. If I were building the GIMP, why would I want them to use my software?
If the goal of the GIMP developers is "kill Photoshop", then I guess piracy is a negative...but if the goal is "make awesome software", then there's nothing pirates can do to impact them in any way. I kinda suspect the goal for most Open Source developers is "make awesome software" first, and stuff related to proprietary competitors is way down the list, or not even relevant. (Again, assuming money doesn't enter into the equation...in which case, the story is somewhat different.)
For what it's worth, I've been a GIMP user since the very first public release (I remember the first mention on Slashdot), and I haven't used anything else in that time...never needed to. Regardless, I think GIMP is clearly one of the great success stories of friendly, consumer-oriented, Open Source software...and it gets better and more successful with every release.