If you want to learn scripting and you have a bit of programming knowlage open up the script editor and in the help tab opem MEL command reference and start to look through that its got samples for each of the different commands and you should pick it up pretty easy.

I use expressions a lot when animating particles or other semi random stuff, also knowing scripting is great for making little commands for rigging.

Mainly I use MEL to put in and pull out data form maya (not 3d stuff facts and figures for further analysis.