Good point on shake reduction

Ivan's point is worth calling out. UNLIKE Canon and Nikon, Pentax puts shake reduction in the camera body. Thus, 30 year old lenses work with shake reduction turned on. And brand new lenses can be smaller and cheaper than an equivalent Nikon and Canon lenses which must put shake reduction inside each lens. There are advantages and disadvantages to each system, as with everything in life, but the original big advantage of putting it in the lens is that it worked with 35mm film cameras (before digital even existed). That advantage for Nikon and Canon is now gone and more of a historical decision they continue to live with.

Mine is bigger than yours, ner ner

"So you have an image stabilised auto-focus 300mm f2.8 then?"

Who really cares? Hardly anyone actually buys lenses like that, and if they do, they probably know in advance not to get a Pentax. It's not exactly sort of lens you buy on a whim.

"If you need to justify why you bought Pentax, fine, but I suggest it wasn't for the wide availability of lenses..."

Look! My camera system has more lenses than yours! It MUST be better!

No. Maybe people buy Pentax (or Sony or Olympus or Panasonic) cameras because those systems have the actual lenses they need, along with cameras that have the features they need.