I was thinking "because you have to load the google page, then click the reader tab then wait for it to load again, which is annoying," but I didn't realize you can add the others individually to the home screen. I just tried it and it works great! I have Gmail and Google Reader on my home screen, instead of the Google icon. Awesome!
Nope, no idea why you'd use a RSS app, Google Reader does the job well and quickly.