I'm mixed on this. On the one hand, I agree with you. Well isolated systems can be messy internally without causing trouble. On the other hand, error messages.

I'm mixed on this. On the one hand, I agree with you. Well isolated systems can be messy internally without causing trouble. On the other hand, error messages. STL has famously awful error messages. I do want to know what the compiler will spit out when I've got a syntax error or a type error when using one of these constructions. Because teaching people new to C++ what "you stuck a non-movable type in std::vector" error messages look like is not a trivial task.