The distortion is around a couple thousanths in the areas around the bolts. The boring gets it within a few thousanths of final bore size so you can do it with the plate. But it's just more work to bore with the plate, take it apart, wash the block, then reassemble and hone it. That's assuming the shop has two plates otherwise it's a lot more work - remember it has to be correctly torqued each time it goes on. Honing is what brings the bore slowly up to the final size. Honing needs to be done with that distortion occurring. Boring simply doesn't.