Heat spots are BAD for plants, it burns them! Mylar don't have anything to do with security issues, it's just the best reflective material you can use, it makes best use of the light you've got, letting very little of it get wasted. If you were to use foil for example not only would it be bad for the plants but around 20% of the light would be wasted, that means if you got 5 cfl's the plants only get the light of 4 of them, you'd be paying for the juice of one light which wouldn't be getting used by the plants at all. It just makes sense to use mylar.