I'm pretty sure they do this. I go to gigs often and most sound tech's now walk around with iPad's/iPhone's during the soundcheck using them to measure levels. I think the problem is more than most venues will have a lot of speakers setup up around the stage (sides, above sometimes too) and that's it. So if it's a big venue the volume is necessary to make sure the people at the back are experiencing it properly too. Sound desks are often centre back of the room so that's where things tend to sound best. If your nearer the speakers you'll often miss things (for example you'll see someone playing a guitar solo and won't be able to hear it, if you go back towards the sound desk it'll be much more balanced).