ORIGINAL: Pippin
I see Au, GB, and Pr mixing corp all the time in a coalition against France.
I think it is much more stronger to operate independant for corps depending on the angle you look at it. Too many problems arise when they are mixed. I also don't like the fact how I am restricted to who I can declare war on because that person just happens to have a corps of two factors left on my door-step, etc.
A lot less restrictions regarding naval stacks. I have no problems with stacking the Spain and sometimes even Russian ships with my French in the channel. Great way to knock out Britain early in the game if he is known to be a trouble maker for all.
IMO, mixing corps is essential for attrition. Not factor attrition, but PP attrition. By splitting the PP losses, it doesn't allow France to focus on taking one country out of the coalition through massive PP loss.
Personally, I can't even see how the coalition could stand against France without mixing corps. If I was France I would focus all my energy on one country at a time and but them into the instability or fiasco zone VERY quickly.
If Prussia and/or Austria loses two or three large attacks (involving 4 or 6 corps) against France and suffers those 2 to 3 PPs for each against all on their own, they will not be able to continue to fight France, hence splitting the coalition and eventually destroying the coalition.
If you mix Corps (2 Br, 2 Au, 2 Pr) and you lose then it's only 1 PP for each person and the coalition can fight and continue to lose battles longer than they would otherwise.
I am surprised you don't do this considering how experienced you say you are at EiA.