Nothing wrong with using amps for show. It definitely isn't the same thing as using playback, not even close. Amps that aren't plugged in don't do the playing for you.
Regarding Periphery they do run cabs and FOH live now but not for show, it's for the people in the front who may be just in front of the PA speakers or just not far enough back to get the best experience sonically.
I don't have any problem with just seeing some rack gear on stage, I'd rather play on a small stage with just a rack than with 4 half stacks behind me taking up all that room. Sure it might "look cool" sometimes, depending on the person, the act they are seeing, the venue etc. but at the end of the day I'm spending the whole show looking at the people, so I don't much care. 75% of the time I don't remember what the backline was at a show I went to, but I remember what everyone was wearing, what guitars they played, where they all stood on stage and so on, because that's what I was watching.