Personally speaking I don't have any problems at all with bands bringing their own engineers as long as it doesn't interfere with the way I'm running my shows (dudes taking forever to setup and check etc). I'm used to it because most touring acts will bring their own engineer. I figure if they know the band's music better than I do then they will probably do a better job mixing if they know anything about running sound (I have only thrown one dude off the console before because he had no fucking clue what was going on, I essentially mixed the show for him).
If you are not the headliner and it is not your show don't expect a soundcheck or any huge addition to the input list that the house guy already has run, or for your engineer to be allowed to change the house graphic EQ. Just make it as easy for the house guy as possible, have your dude help him out on stage, be nice, and you shouldn't have any problems.
The only places where I've been told I wasn't allowed to mix were small bars with tiny systems and owners who "only let their engineer" mix on the system to "prevent damage." Places that where touring acts will never, ever play. One was a christian club that is now closed, the other was some empty, hole in the wall bar in Chicago.
