Here's my $.02:
For the MK, EE can be a huge benefit, particularly if you're interested in the FL/TL rides that are open; then at rope-drop, you can hit the Mountain(s) of your choice. However, I suggest that you plan on leaving at or before noon, as the MK gets very crowded on an EE day.
For all the other parks, I suggest NOT going on EE day. They usually only have a few rides open, and the crowds drawn by EE usually out-weigh any benefits.
IMHO - YMMV