“Reply All” is the bane of my existence. I would like to spend a little time today complaining about it, but it might be worth my while–possibly even worth your while– to try to understand it first.
The first problem is “reply.” To whom does one “reply?” To one who has addressed you, of course. And who is comprehended by “you?”
If you and I are having a conversation, I say something to you and you reply. I have addressed you and you “fold back” the conversation to me.  That clarifies “reply.” But if I address a group—which is presupposed by “all,”—anyone in the group might reply to me directly. “Dear Dale, thank you for your note.” Or anyone could “reply to all,” in which case you send your thanks to me to everyone. As a practical matter, you are “replying” to me (because I have addressed you) and “informing” everyone else that you are replying to me.
That is the silly shallow end of the pool. Let’s go deeper.
Let’s say that I propose in my email to the group, of which you and I are both members, that in every instance of a population being served, the population be referred to as “constituents.” They are not patients  as those served by a medical doctor would be or “clients”  as those served by a lawyer or social worker would be or “customers,”  as the regular patrons of a business would be. No, these are constituents, a distinctly political term. So when I propose that the term to describe this population should be “constituents,” I am proposing that the role be politicized.
I proposed this to everyone. If you respond to me, you really ought to respond to the group. To use the language I invented above, you really should “reply” to me and “inform” everyone else of your view. You are of the populist wing of the group. Everything that treats transactions as if they are political and best responded to by the broadest possible set of the people who will be involved, the better. You will and should respond to me and inform everyone else that “constituents” is exactly the right word for the people we are talking about.
Then someone from the other wing—we will have to call them “representationalists,” the word “republican” having become either archaic or misleading—replies directly to me (and indirectly to the populist) and informs everyone else of his or her views. “No, no, that is a terrible thing to do. Politicization of this set of transactions will: a) ruin the transactions, b)it will elicit an electoral selfishness from the “voters” and c) it will cause the public spirited voluntarism of those now doing the work, to atrophy.” In your “reply to all,” you are directly opposing my proposal and the agreement offered by the populist and indirectly trying to persuade everyone else in the group to oppose my proposal. You are, that is, “replying” to the populist and me, and “informing” everyone else.
Others who follow, are “responding” to those in the conversation and informing everyone else. The proportion of people who are properly being responded to becomes a larger and larger fraction of the original “all.” The proportion of people being “informed” becomes progressively smaller. This is just as it should be when an issue is being discussed.
Even short comments like, “I agree with Dale’s proposal” [or “not”] belong in this chain. Or even, “Dale says that what we call the group of people is the crucial thing. That is evident in his proposal that we pay attention to the name. But the real issue is…” That is not a reply to my proposal, but it is a reply to me: the reply is that people ought not to reply to me on the question I raised, but on another question entirely. Fine. Even better than fine.
But what if we are only scheduling a meeting? Can you meet, staying, of course, six to ten feet apart, next Thursday at 2:00? This a convener question. Replying to the convener makes sense. No one else needs to know that that is the time you are scheduled for dialysis or you are going to your beach cabin or your grandkids will be here then for their monthly visit. THE CONVENER NEEDS TO KNOW THAT. I do not. And furthermore, I don’t care. When the convener has heard from enough people to know whether to schedule the meeting, he or she will let us all know. “Reply to all” gives us all the information that only the convener needs.
It reminds me very much of the old party line.  It was always possible, but never certain, that you could call someone else and have a private conversation. A listener could always “reply to all,” that is to everyone else on the line at the time. In the present case, a proposal has been made. “Use the word ‘constituents’” is a proposal. “Let’s meet at 2:00 on Thursday” is a proposal. Everyone who has something to say that bears on the proposal—yea or nay—is part of the conversation. Everyone else is just on the party line.
So…what does it really cost? There is no way to say and I’ve read people who have tried. Instead, imagine that your time is worth something. Every time you get a message in your inbox that might contain information you need, then you need to find out whether it does. If it is a “convener only” reply, you just wasted some time and some focus. If it is the twelfth “I agree with Simon” message, you just wasted time and focus. You have to keep your inbox cleared so you will have a chance to notice new messages. It really isn’t an option. You are therefore obligated to check out these “messages” as part of the cost of keeping your inbox empty.
In the absence of any real information about costs, let’s say it costs twenty-five cents to check a pointless message in your inbox and delete it. And let’s say you get twenty such messages a day. Every day. By my calculations, that is $35 a week, roughly $140 a month. Remember that I am including in that all the valid messages, the ones to which I should respond (and inform) so it is not as if I am getting no value for my $140 a month. It is, rather, that I am getting no value for the fraction of those 20 daily messages that ought not to be there. 
Of course, it costs you, too. Every time you decide to respond to the sender, rather than to all, you run the risk that everyone else will think you are being secretive and are “excluding” them. It may always seem to you that the more prudent choice is always to include everyone. That way no one will feel left out. Always safer to include, you think to yourself. And what does it hurt?
This essay has been an effort to answer that question,.
 Etymologically, “reply” is made of the same materials as “replicate.”
 Literally, “sufferers.”
 Etymologically, “leaners on.” The Patrician/plebeian relationship in ancient Rome is the controlling instance for this usage. The plebe “leans on” (clinare) the patron.
One who as a matter of regular practice, buys from the same tradesman or guild.It is a contraction of the Latin consuetudinarius.The “regular practice” part of this history shows up in our “customarily,” which may have nothing at all to do with purchasing goods.
 This is back before “party line” meant parroting the talking points the party distributed for your use.
 And that’s just the “reply to all” part of the problem. I keep getting advertising from people who have agreed to honor my request that they stop sending me things and then begin again. That’s a problem, but it isn’t this problem.