Sam, the way I explained it to my bandmates, (and we'll see if this works out at the first gig I use all this new gear at) is that it isn't that I don't trust them with my equipment, but that the reason I am setting a password on their device that only I know is because I don't want them accidentally messing with the main mix. I do have one guy who subs for us on occasion (a rhythm guitarist) who is a talented mixer, and you know what? If he wants access to the mixer as well when he plays with us, I will give it to him. He knows a helluva lot more than I do about mixing.
However, when I explained the reason I will be locking them out of the majority of the mixer functions and ONLY giving them access to their monitor mix, they completely understood. Look, they don't want to be ANYWHERE NEAR the house mix or their fellow band member's monitor mixes. So what I told them had the virtue of being absolutely true. Because if someone thinks they are pushing faders for their own mix and ends up on the house mix, they could mess up a good mix or cause insane feedback. lol