I have been to Phoenix several times and the dry heat there does not compare to the 90% humidity we see here in KY in July and August, with a full face (I wear glasses) you could not see where you were going.
I've lived all over the US including the Southeast. Hot is hot. I understand what you say about wearing glasses as I too wear them. The helmet will absorb your sweat and a slightly opened visor will provide plenty of airflow.
I am of the opinion that the better you can see and hear it increases your reaction time and a couple of extra seconds makes a real difference. A full face prevents you from seeing and hearing what is going on around you. We have a group of ****** rockets here that meet every Sunday and they come padded and with helmets, but **** they ride like idiots and one of them crashes almost every Sunday. I am older and slow down for every car coming to an intersection. They guy that pulled out in front of me waved at me before pulling out in front of me. Now if a car is waiting to pull out I slow down and have an escape route.
Again, I'm sorry if this sounds opinionated, but that's just bull****. If a full face helmet is the difference between your being aware of your surroundings and not being aware then your senses are sorely lacking.
I'm 61 years old and have been riding for 49 years. I've owned just about every type bike made over those 49 years including ****** rockets. Just because those guys are fools, don't discount the gear they're wearing. I've seen more than one t-shirt wearing cruiser rider over cook a corner and suffer the consequences. Have you ever seen a Moto GP rider crash at over 180 miles an hour? They often walk away. Its the gear they're wearing.
If you choose to wear less than the best gear available because its not cool to look 'all padded' then that's your choice. Just don't pretend that its somehow safer. Its not the car you see and slow down for that will get you, its the one you don't see.