> "If you want to use performance critical software that pushes the limits of your machine, download it, use Java or whatever."
This is a bit too simple. The web is becoming a platform for rich interactive applications more and more. Nowadays we can use tons of applications without downloading and installing any software. Most people would agree this is a good thing.
Maybe NaCl or even some bytecode alternative have their disadvantages, but they solve a very annoying problem for sure.