It's an excellent lens protector, preventing dirt/dust/pollen/rain(Pretty useless without one in the rain!) getting onto the lens, branches,brambles scratching the filter etc. I can imagine my lens getting covered in all sorts quite quickly without one! It also provides a clear 'tunnel' to shoot through when poking through grass/bushes etc. and cuts down on 'gardening'!
Also I find it does prevent lens flare quite a bit when shooting into the light and on occasions I need to use my hand as an extension of the hood because sometimes the one with my sigma 150 just isn't long enough. I notice the new 150IS has a longer hood for use with crop sensors which I was very pleased to see!
I can't imagine that the hood would scare off the butterflies. It's a minor addition to the lens+camera + photographer already in front of it! I can imagine waving your hand about in front of it trying to mimic the effect of a hood is going to spook it more! Having someone else wander over and wave their hands about even more so!!
