Dave, the only drawback with using the onboard speakers as monitors is that we tend to have them turned up louder than the PA system
Oh, I'm pretty sure I'd know it if my mains went off!

My hearing is still 100% - bugged the crap out of my high school students that I could hear all those annoying high frequency tones they tried to torture us with on their phones.
I attribute my stellar audio health to never being in front of loud mains all these years. Monitors were always a priority for me.