Apple initially introduced this feature with OSX Lion. When I saw it I immediately tried it then just as quickly forgot about it. The feature is still there on OSX Mountain Lion, and I only recently realised that I have actually been using it all the time without having noticed the point in time when I changed habits.
Your Mac, just like your Windows machine I guess, is full of nice features that you seldom use. If they were truly metering the usage of all the features that get thrown on new OS releases, I think it would be astonishing to see how much bloat creeps around unused.
Safari Reader isn’t such a bloated stuff, it comes in really handy for folks who frequently read articles on the web. Look at the two pictures attached below, it is immediately obvious which one you would prefer to see, that’s how useful the Reader button is. It is not available on all sites as I’ve experienced it, I didn’t try to find out why, but if you are aware of it, it can nicely enhance your reading experience without a sweat. That’s why I use it all the time.