I think the issue is that many people would react, but when they read the emails in the thread one by one they might decide to respond before getting to the advice. Similar to what happened to me lately when I sent out information that I can't make the meeting, then some time later that I'll be there, but have to run away early, then something else again (over a couple of hours). When the recipient came to the office, he rescheduled the meeting 3 times in 5 minutes instead of reading all the emails first.
Is it bad though? Should we really expect people to process all emails at once and only then act on them?
A reply-all debacle occurred where I was a consultant a couple years back (at a quasi-government corporation with 5000 employees).
In this case, "Should we really expect..." is like saying "should we really expect people to read a dozen emails before sending 5000?" Yea, I think so.
One of our consultants wrote off a sarcastic reply-all email in response detailing what the reply all button actually did as if explaining to 5 year olds. The client demanded that this person be fired.
Is it bad though? Should we really expect people to process all emails at once and only then act on them?