Larry, there's actually a reason why a sound guy would have headphones on, but if he kept them on a lot of the time, it's indicative of an OC disease. The cans let you solo an instrument to see what it sounds like by itself. If someone is busy trying to get each channel to sound "optimum" they will spend the entire gig in the cans tweaking each channel and never listening to the overall mix.
You solo something when you hear something funny or realize that something is off and you're looking for it. For the most part, you should be able to go back and forth between soloing an instrument (let's say your rack tom) and then going back to the mix in the air to hear if the eq cuts you made cleaned things up. Solo'd the tom may not sound very good, but in the overall mix, it may have distinction and allow your fills to be heard against the keyboardists left hand or whatever.
Many bands I've done sound for have everyone going at once. Often in the same register or sonic space. Most of the time, you can chop out 90% of the comping instruments that are consistently banging away. You'll still get a wash of the chord or harmony that's underpinning what is going on, but it won't overwhelm it. If you find the occasional musician who is spaciously stabbing comps in where it fits the arrangement, then you can leave them up.
The trick is recognizing when someone needs to be brought back up for a solo or fill part that is germane to the song. Musicians to mix themselves on stage are so much easier to work with. Get a balance and cut what you need so that the various instruments aren't getting muddy in the room, and sound as much like they do on stage as you can, and then sit back and enjoy the show. Unfortunately, that happens rarely.