GIMP

I see very little mention of GIMP anywhere on DP. Why is that ?

Being a Canon user, I have previously found ZoomBrowser adequate for filing, elementary mods (such as red-eye) and slideshows - but I'm starting to want to do more than ZB offers. I already have DPP available to me and have started using that.

Given that I do very little post-processing yet, but am expecting to do more, is there some reason why I should spend money on PSE and/or Lightroom, rather than investing time in learning how to use DPP and GIMP ?