I did a "quick mow" with my FEL mounted for the first time in 5 years just before July 4th. Eeesh...couldn't get close to anything and made the tractor 'bound' uncomfortably. I could only imagine doint the same with the BH attached...I would try but for the fact that I use the bagger and it is even more of a pain to remove the bagger boot from the MMM than to just install the bagger.
As for the original question, Mr. BXpanded's explaination makes sense, but that would seem to mean that ANY operation of the 3PH hydraulics while the BH is installed and the RPM is up would/could potentially damage the valve in question...don't know why having the MMM installed would make a huge difference other than the fact that you'd then have a REASON to operate the 3PH hydraulics.
Therefore, it would seem reasonable that you could leave the MMM installed as long as you don't use it with the BH installed and if you DO use it, turn the RPM down before you raise/lower the deck.
Simply stating "No MMM with BH" is an ambiguous and confusing rule. Having a reason would allow folks to avoid whatever condition it is that damages whatever it is that gets damaged.
