I have already posted, but thought I'd give some perspective from someone who has perpetual GAS (gear acquisition syndrome

).
Back in the day, Palm and Pocket PC devices used resistive touch screens. Because they were resistive (i.e. needed pressure to register touch), they were often made of far less durable materials like plastic and often subjected to much more abrasion (i.e. styli). A screen protector even then was not necessary, but if you didn't want your screen to look like you'd tried to carve your initials into it, a screen protector was a wise investment.
Coming from several Palm and Pocket PC devices, the iPhone was my first capacitive touch screen device with a durable glass screen. Because of my history with those earlier devices, what was the first thing I did? Slap a good quality static cling screen protector on there! I wasn't going to take any chances!
Even if the glass screen of my Evo is scratch resistant, that is a far cry from scratch proof. It is just as easy to express disbelief that a phone which has been subject to abuse and carried in pocket with keys is not scratched as it is to express disbelief that a phone carried under the same circumstances is scratched. Everyone's different and everyone has a different level of OCD when it comes to their personal devices.
My point is, the screen protectors on the market today by companies like Seidio, Brando, SGP, Pocket PC Techs and others are all high quality products. It's hard to believe that a company that knows it's pulling the hood over your eyes would invest so much time to come up with high quality products if they didn't believe in them. It's up to the individual as to whether or not they want to install one. Me personally, I just put an SGP ultra crystal on and I love it. Great fit, feel, and I love that I don't have to baby the touch screen. Would it be fine without it? Probably. Do I want to take a chance on scratching the screen? No. Do I give up anything by having a screen protector on there? No. And I've tried several on the Evo already- Clarivue, Seidio, and SGP. They're all good, although slightly different in design, but made of high quality materials. As you can tell, I'm a little OCD on this subject, but that I also means I have a lot of experience with them on multiple devices- they are not BS.
FYI, I am not a paid employee of any of these companies. The opinions expressed above are mine and mine alone.