Playing without them in my monitors is horrible. It's has happened to me due to breakdowns.
We may set up differently than you do.... you may have your amps behind but a little to the side, at least a little maybe? Well, maybe not... but our guitar players set up their amps directly behind them. Bass player is directly in front of his amp... as is the lead guitar player and the rhythm guitar player. Everything goes through the mains which ARE to the sides, obviously.
Set up like that I have no need for monitoring the lead guitar because I think the sound that comes out of the back of an amp is nearly as loud as what comes out of the front. Since our lead guitar player sets up just to the side of my bass drum his volume is overwhelming withOUT being in my monitor. In fact, it's the opposite problem... when he turns it up, which he does occasionally, especially if he gets very excited, he will drown out everything else... I won't hear the bass which is in my monitor and I can just barely hear myself on the drums. My bass drum not at all.... IF he cranks it up. Fortunately he doesn't do that often anymore as we scolded him severely for it... no one else in the band could hear themselves playing. Now that's NOT a criticism of volume... I LOVE volume... but balanced volume... not one instrument 5 times louder than everyone else.
I'd have to say that we never did master the perfect setup. We did very well most of the time but I doubt if we ever hit perfection.
The best system we ever played was the concert I talked about here.. in Oregon just a few years ago. Trace Adkins, Dwight Yoakam, some others. That was perfection... or at least as good as we ever had. They didn't allow us FULL use of the sound system, however.

I think they didn't want any opening group matching the volume and quality of the headliners. Not that we could have.

And just to add.... over those many long years I could never get used to headphones, or earbuds... I hated them while playing. Which is really too bad because it would have solved the 'hearing' a balanced sound problem. - JamesJM