Well, I'm definitely no expert in the uses of Pymel, but there are a whole lot of benefits I have seen in using it. While python in Maya is kinda nice, it's not object oriented, so if you are going to use Python to it's fullest extent you have to take the time to build all your classes and modules from the ground floor up, and if you want to do this properly you'll have to take a pretty hefty amount of time planning it out and executing it. (especially if you want to use super classes and inheritance) Pymel solves that problem by having all the classes built already so it functions in an Object Oriented manner, while also allowing you to build your own classes as you need. Also, since it is a tool born out of production, it already has a wealth of extra modules built into classes that Maya doesn't have built in natively, I've noticed this a lot in the GUI portion of pymel.
I'm pretty sure that it has to be on all machines that intend on using the tools built in Pymel, but that isn't very difficult to do. Hopefully that gives some insight. I'm pretty sure more Pymel fluent people here will know more. Brandon L. Harris -- http://groups.google.com/group/python_inside_maya
