I hear that argument but let's think about this one a bit.
If the dive doesn't go according to plan then either you have enough gas to complete out closed loop or you don't.
If you don't, seeing the needle go to zero on the O2 pressure doesn't add anything except stress. You're still screwed and going to have to go SCR off your bailout gas at best anyway.
This is kinda like the argument I've gotten into over cave diving when you donate gas. There's a "dogma" that says you show the donee your SPG so he knows how much gas you have, and you look at it too.
Why? Either you have enough to get out, or you don't. If you don't you're dead. If its marginal knowing 20 minutes ahead of time that you may be dead is likely to make it happen!
I've had a "boom" in an overhead on OC. I got the situation under control and started exiting. I was solo, so it was "all me buddy." I didn't look at the gauge. Either I had enough gas to get clear and to where I could breathe my deco bottle safely, or I did not. If I did not knowing ahead of time wasn't going to suddenly make more gas appear in the can!
I can see the argument on the diluent bottle since there's a whole host of things that can screw you there, and some are "normal" (e.g. your mask gets knocked off, etc). But on the oxygen bottle I'm less convinced, and there IS a downside to the gauge and its hose - its a significant reservoir of gas which can get dumped through a stuck-open add (either solenoid or manual), and too much O2 in the loop is bad.
I'm trying to weigh the ying and the yang here. Its not hard to run the SPG hose up and over the shoulder, but I'm not sure its a good idea for the O2 gauge for that reason.....
PS: Logic wins over dogma (e.g. what someone told you.) They should be able to defend their position, and if you learned something in a class so should you!
