I am one of those strange people who enjoys reading the Appendices. I like factual books - and the more strictly factual the better, as shown by my determination a few years ago to learn the Periodic Table 'because it was there', and I love a good Atlas or encyclopedia. Hey, call me a nerd.
When I read the Appendices (which usually means jumping in at random to see what I can find in a spare five minutes) I often get the impression that Tolkien himself loved this kind of thing. I wonder if he had piles of notebooks with chronologies and family trees and such like stashed away which he would guiltily tinker with when he was supposed to be getting on with writing his story - Tolkien's equivalent of the garden shed, where men go to count nails and potter about. In the new Scull/Hammond book, it says that Tolkien had the idea of putting all the appendices in a separate volume but they were later put into RotK in a much condensed version; he was still tinkering with the appendices in March 1955 and the publishers were getting restive about getting RotK out and in to shops.
I can't imagine the books without them; until relatively recently, single volume editions lacked the full appendices and I know of people who have read LotR several times but never the appendices! Do we need to read them though? They include a lot of information that isn't strictly necessary, but what they do give us is a taste for further stories about Tolkien's world. Some (most?) of those stories were never written, and that's where RPGs and fan-fics come in - the appendices must be a huge source of inspiration for writers and gamers. And though I find there is little to argue about in them, as they consist mainly of facts rather than prose, I would be lost without them as they add a sense of richness and depth, of history to Middle-earth.