When I do landscape I am always wanting a wider lens than my 17mm on 7D, equivalent to 27.2mm FF. As you have this as your special landscape kit how do you cope with 28mm when landscape is about the only thing you cannot take a step back to compensate for lack of width?
As someone who shot a hell of a lot at 11mm on APS-C (so, 18mm on full-frame), going wider has its drawbacks. You tend to want to include too much and come up with weak compositions. Sure, there are times it is the only option for a dramatic shot, but, on my last trip, I found myself shooting in the 17-30mm range a lot more than 11-16. Heck, my 3rd and 4th best shots were at 85mm and 200mm.
Doesn't say where he shoots, but, if I was anywhere near mountains, that 28-300 combo would be perfect for variety. Plus, you can always stitch together a few shots at 28mm to create 20mm. Big advantage to no lens changes as well, if you are in the rain, wind, snow, dust, etc.