RE: Why do you use a rangefinder?
Come on Rybo - how would you know that you're not shooting beyond 20 yards unless you had a rangefinder? UmmmDuh. [8D]
Seriously though, for the guys who keep the shots short, there's no real need for it. But, if you're a one-pinner, and would push the limits if you had the opportunity, it never hurts to know your yardages definitively. I don't have one. If I had the spare cash laying around, I would. But I don't, so I don't.
People have killed deer for years without 'em, so by no means are they a necessity, BUT I will say this much:
There aren't many things on the market right now that can actually make the difference between you filling a tag, or going home skunked, but the rangefinder is one of them.
No matter what bow you're using, no matter how expensive your sights are, no matter what kind of cresting tape your arrows are wearing, or what broadhead you have - a misjudged35+ yard shot where you're +/- 10 yards will probably result in a long, sad ride home.