"I've seen people say on here that, when editing ROTS, they want to preserve the Vader-is-Anakin reveal. Why?"

My take: (and obviously echoing others) to maintain the element of surprise/doubt. Back in the day in the three years between ESB and ROTJ there was rampant speculation on the playground about whether Vader was lying in order to manipulate Luke, and even outlandish talk in Star Wars weekly that Boba Fett was Luke's father and that's why he fired a warning shot rather than a headshot which of course we are all sure he was capable of. It really didn't sink in as for real until the reemphasis in ROTJ by Yoda, Ben, and Vader himself.

Personally I don't have much time for the PT, though would definitely seek out Adywan edits.