Dial in some reverb, crank up the distortion (edit: I like Crunch or Super Crunch for lots of energy and character) and play several bars of open G-D-A-A in a Rocky riff style, ending the final bar of that riff with an emphasised sustained E chord, and tell me then if you think there's any use for open chords in Rock. = ]

May not be to your generational taste, but a prime Classic Rock example, the rhythm guitar riff in "Satisfaction". Watch Brian Jones fretting hand in this clip, you can see he's playing open down the bottom of the neck. E-A-D or E-A-D/A. Inarguably rhythm for that song it sounds better/right down there too.