Shirley's albums with Maiden still need to pass the test of time. Piece of Mind was released more than 30 years ago, and it is absolutely not dated. Mckindog's opinion makes sense to me. Birch gave every album a distinctive sound, each instrument could be heard perfectly. Shirley would like the band to sound "live", but he associates bits and pieces recorded during different takes, which causes different problems. I think he might have helped the band to last longer, as this recording mode seems to motivate them more and to be less demanding, but I don't think the result is equivalent to the traditional way of doing things.
Of course not. The record is an Iron Maiden's, not Shirley's. It is normal that the final word goes to the artist. He knows how he wants things to sound like. And unless I hear the two conflicting versions of a record, I wouldn't allow myself to say that he has commited a mistake.