It's not even always about "can we spec this?" or "do we even understand the problem?". But when you get into the real world on a non-isolated system, you have

It's not even always about "can we spec this?" or "do we even understand the problem?". But when you get into the real world on a non-isolated system, you have to deal with invalid data, unexpected input, network failures, hardware failures, software bugs, security probing, user errors, memory limitations, timing issues, remote system failures, corrupted data, network failures, data storage failures, network failures, network failures, and network failures. Did I mention network failures? Also, as beautiful as side-effect-free programming is, any non-trivial program will have side-effects outside the program itself. And your beautiful functional code must account for that.