Your original statement lacks any nuance except "E are bad because no pure functional programming". Any attempts to clarify resolve to basic "it's for the good of the whole program".
Whereas easy-to-use concurrency primitives, effortless parallelisation and concurrency, and even the most basic stuff like ability to put a logging statement anywhere in the code without re-engineering half of your program trump whatever imagined advantages of pure functional programming you may come up with. Any day of the week, and twice on Fridays.
In theory, theory beats practice. But in practice...
I did not intend to write an essay of why PFP would be great to have in Elixir/Erlang. I'm just posting my opinion here and I'm happy to explain it as you can see from my answers.
> ability to put a logging statement anywhere in the code without re-engineering half of your program
Let me ask you a question: does it change the semantics of your (whole) program if that logline, that you are talking about, is not executed for some reason - or if it is executed more than once?
If your answer is "it doesn't really matter, might at most be a bit annoying but it's just a log, no stakeholder of the program can ever notice" then there is no problem with putting this logline into the program. No need to re-engineer anything.
If, however, this logline is critical and will be e.g. parsed and used by another system and actions might be taken due to it, then I would argue it is good if you are forced to consider the potential impacts to your program. If that means that you need to re-engineer half your program then there is a good reason for that, since the potential impact is huge. Such a thing has never happened to me in many years while working on different kind of applications. Sometimes a couple of 100 lines need to be rewritten - that is the max that I ever had to do. And indeed sometimes this rewrite in fact caused me to find and resolve problems that would have otherwise been introduce by accident.
Because yes, it's a fact that in a "pure functional program" you need to re-engineer half of the program if you need to put the log somewhere where it's "oh so pure", and where you didn't need logging before [1]. Or thread IO through the entire program to begin with.
> Sometimes a couple of 100 lines need to be rewritten - that is the max that I ever had to do.
Where in a pragmatical language you just add `Logger.log` or equivalent
[1] Spare me the pontification of "if you need logging, you're doing something wrong, this must be covered by tests or type systems". There are things you must log like metrics, audit logging, tracing values through the system, and it is a 100% certainty that you will add logs to places where no logging existed before.