If I rode like some of you guys at 75-100 mph 99% of time I might consider wearing one then but most of my riding is at 60-70 mph.
I suspect that bouncing one's face off the asphalt at 60-70 will still do a fair bit of damage. But, as others have said, its your head and its your decision.
As to the heat, not many places get hotter than Phoenix. I've found that a 'quality' full face helmet like the Shoei X-11 and Schuberth R1 both have excellent ventilation. A short screen or preferably no screen and a good helmet will provide sufficient air flow.
As to the rest of the gear, well if you've seen someone in shorts and a t-shirt contact asphalt that's been baking in 110+ degree temperatures all day, the propect of years of skin grafts suddenly out weighs a bit of sweat at stop lights. A light colored, armoured mesh jacket will just as comfortable as that t-shirt when you're moving.
Sorry for the sermon, but when ask opinions you occasionally hear from opinionated old farts like me.