Screenshot of a posting elsewhere of a slide from a very recent ASBC/MBAA presentation. In case it's illegible, it says best practice is rehydration in either sterile, salt-adjusted brewing water or diluted (2°-5°P) wort to avoid cytolytic shock. Sounds like ancient history, doesn't it?
Meanwhile, studies have long shown that no matter how you pitch, the fermentation ends up at the same goal at the same time. Wonder if it's not the same principle as vitality starters. Sure you kill some yeast, but what's left is so dang healthy, who cares? I've always worried that rehydration could be counterproductive, because the delay involved could allow the dry yeast to consume all those wonderful reserves the production process endows them with before they get into the wort, disrupting their metabolism.