I wonder if there is an exception for execs. At a previous employer it was quite common for a big piece of news to be announced by a senior exec and then have the CEO and other execs reply all in a big storm. Definitely more than 5,000 FTE on the list.
There often are exceptions for execs for rules like this, which can cause their own sets of problems. I was at Microsoft when Steve Ballmer sent the company-wide email announcing he was going to step down; Reply All was disabled for most people, but some poor VP in MSR, who I guess was high enough up to be an exception, accidentally reply-all'd the whole company with his positive reaction to the announcement. That guy must've had an awkward day.
I find it sometimes interesting to skim through trivial (praise/reaction) and nontrivial (response/correction) reply-all threads to gain a better understanding of the social dynamics, attitudes, etc. across different departments in a company.
In other words, while they aren't always useful for immediate work, they sometimes help with metawork.
