Hi, Angela!
I think this is a great question, and it really got me thinking this morning.
Since an E-define ("English define") has no steno associated with it, those entries could never be placed into any dictionary. A J-define ("job define") has associated steno, which means you could later merge those entries into your PD, or you could share the job dictionary with another reporter. So I guess those are the main advantages.
There are times when there IS no associated steno with a particular word (say, if you typed it in, or if you performed a replace) and you HAVE to use an E-define instead of a J. So both types of defines have their place in editing, so I'd never want to be limited to one or the other.
I use Eclipse, and our E-defines are called "trash globals," so I guess that means they're not meant to be saved. <G>
Deanna ;-)