I agree, should you always test the reducing function separately outside the reduce? And if so, what's the real benefit of not mutating accumulator? Seems silly to just copy a transient value which is immediately discarded and then do it for all reduced elements. Does the original reference to the object even stay during the iterations? Hmm, I guess it does.

To me it seems weird to be so puristic about a simple reduce-function, which by-design leads you to mutate the accumulator. I mean for object-accumulators it definitely is just a massive waste to not to just mutate the argument directly without copying. Although as a disclaimer I have to say I am big fan of simple code, were it FP or not. So if you have a messy reduce-function I guess having it immutable makes it a somewhat easier to manage.