I'm not sure why they would be like that.... especially when the entire theory behind in-ears is to block out the stage and listen to "what you want" at the "volume you want". If anything with in-ears, I find singers (more than players) will hold back compared to having a wedge. Too much of themselves, and something they might not put the emotion and energy in that they used to. But that's the opposite of what you are describing! Those mixers are getting pretty common for many groups. I've toured with two, and know several others that use that setup. When they get to a venue (including a festival stage), they patch in the mixer for monitors and the band it ready to go. Sound checks are getting to be a thing of the past for monitors as long as the band is consistent with the levels they feed the board(s).