I'm not a heavy book reader though I am a heavy reader. The only advantage of the larger iPad is its screen, but even I can live without it. The screen isn't that much smaller either, but try holding the regular iPad in your hands for even an hour.....it's too heavy. And I can read the screen just fine....I have it sitting in my lap right now outside my doctors office while waiting for them to open. I guess if have vision issues you might have an issue....but at read just fine at this distance....even without retina.
Weight is a factor for me only on the go, which is why I got a Samsung Galaxy Note 5" phablet nearly a year before mini iPad came out. For me, the Note is more portable. At home, weight isn't a factor, because I use a stand. (My absolute favorite: Amazon.com: LEVO Deluxe iPad Floor Stand for all iPads Galaxy Xoom Nook and Other eBooks and Tablets: Electronics.)