I don't even know what ES is used for. What does it do?
It's the "embedded" version of OpenGL, with some of the legacy stuff removed and other stuff tweaked for fixed-point operation (since a lot of embedded/mobile CPUs are not great at floating-point math). It's mostly used on mobile platforms (iOS, Android, etc.). WebGL is also based on OpenGL ES rather than regular OpenGL.
I was more interested about stuff that needs ES. So am I missing anything is I build Mesa without it on a desktop PC?
Right now, not really. Most desktop oriented software that support OpenGL ES has a compile-time switch to use regular OpenGL instead, and most distros default to regular OpenGL.
However, as a lot of mobile/embedded hardware has OpenGL ES drivers, but not full OpenGL drivers, some distros, especially those trying to compete on the ARM architecture (such as Emdebian and Ubuntu), might change that policy in the near future.
the GPU still locks up and tries to reset (like I experienced on Ubuntu 11.04 where it was successful to reset but did resets pretty often)
but seems to fail (sometimes it reset the gpu 1-3 times - screen goes blank shortly and I can switch to tty for several seconds and maybe back but then the screen content locks up)
at least it doesn't hardlock anymore (where it couldn't manage to reset the gpu)
I can save the state of my work via magic sysrq key and reboot or shutdown
* it's probably NOT entirely related to Chromium browser but it happens most with it while browsing through github
* it once happened while bringing an minimized window to the front (magic lamp animation and it got stock in the mid of the animation, mp3/music was playing and looped for that moment (1/2 - 1 second loop)
* another time afaik I had switched apps around and wanted to open up an app from awn (avant-window-navigator), clicking on an app icon where it locked up
each time this was with compiz-fusion + gtk-window-decorator enabled
static application switcher, no wobbly windows enabled (these enabled probably trigger it more faster), ...