they're just there for extreme heat situations.
lets say you're towing a trailer, on mountainous roads, its 100*F and your A/C is on.
Thats gonna get PRETTY. DAMN. HOT. back there...hot enough to melt the plug boots? Probably not, but maybe if your car starts running lean for some reason, and your manifolds start glowing
good luck getting melted boot off.
I run them just because there is probably a reason they were put there in the first place, otherwise GM would not have spent the money putting them on there...it'd be cost effective to just leave them off.
oh yeah, and why didn't cars run them before these? probably because they were RWD, and RWD manifolds probably stay a bit cooler being that they're surrounded by more air than a transverse application.