You are a 100 percent right. Why not have it all lined up nice from idle till it starts distorting.I agree also that under extremely high RPM the block will distort. But just tightening the head bolts will distort a block also, at any rpm. So if you bore and hone without the plate on that simulates the head distortion, then it will be different with a head bolted on after boring. I'd prefer it's round from idle to high rpm's, than out of round all the time. Either way it will distort at higher rpm's, so why not get it right at lower rpm's?
Quality machine shops have been using boring plates for decades on high performance builds, and I've never had an engine bored that the shop didn't use one during boring and honing.