Just because you can see through it doesn't mean it's transparent to IR light. The stock glass has additives in it to help reflect IR heating. Adding an additional film to it may help a bit more but IMHO its more psychological than practical - if it's darker inside, you'll think its cooler which maybe is a good enough reason to add tint. I hate how fragile the stuff is, how hard it is to see out at night, and how some films distort the view through them.
'03 KTM 450EXC
"If you know what you're doing, then you aren't learning anything." - A. Lincoln