Note that this is from Nov 2011. Since then there's been a massive discussion about handling requires in a multi-thread environment. A lot of the problems with autoload comes from the fact that requiring isn't thread-safe, so if that gets fixed (global lock around requiring files) autoload won't be that bad anymore and might live on for another release.
It is threadsafe in jruby (as of 1.6.6)[1]. It is a decent feature, and it would be great if matz would reconsider.