Baloney. It has single window mode, and exporting as JPG/PNG/etc. rather than saving encourages the use of the XCF format which is the equivalent of Photoshop's PSD. That's about a 15 second learning curve.
It amazes me how many people are ready to talk smack about GIMP citing the dumbest examples of why it is "bad", "bizarre", or "doesn't listen to users".