If you plot the frequency response of the small matching transformers or volume control transformers, you will see some roll off at the extremes and if you run a square wave through them, there will not be much left. I would not want to wire my $tate of the art $peaker$ through a garden variety autotransformer volume control -- 70V or whatever.

That said, given the alternatives in large commercial systems, 70V sounds great and results in a manageable system at a practical cost.

While I tend toward the high end of things, the 70V commercial systems that I've done have thrilled the customer -- I don't use cheap speakers. And, I'll use a dBx processor to limit and compress the life out of things. To an audiophile this is a sin beyond description, however, there is no point in attempting to run a high dynamic range into an office, restaurant, or bar. The customers don't know why things sound so good (it must be magic), but it's the best restaurant sound that they've heard.

The bottom line -- use technology that is appropriate for the problem being solved.