That's interesting. I bought a Kindle for my dad, who is an avid reader (1 or 2 books a week), about 3 or 4 years ago. Last time I had checked he had moved to the Kindle app on an iPad (and not the mini) as his main reading device. He told me he doesn't use his Kindle at all any more.
Confirm the same - both my parents (avid readers) use iPads with Kindle app or iBooks to read, despite having access to my Kindle3.
From a elderly reader's standpoint, the iPad is like a Kindle DX but a) heavier, b) with color, and c) with tons of other apps. For whatever reason, eyestrain of an LCD doesn't bother them in the least.
I wonder if it also depends on what kind of reader you are. My Kindle is great for linear reading, but I tend to skip around books absentmindedly, and turning pages on the iPad feels so much better.
My dad prefers his full size iPad to the Kindle too. For him the backlit LCD is a feature, and the brightness and contrast on the LCD is better than the kindle display. My eyesight is better, so I prefer the Kindle for reading text.