I'm not about to join the FP circle jerk, but well-executed immutability can reduce memory bandwidth requirements rather than increasing them. This is because often in large C (and Java) codebases, the uninitiated will copy large structures and buffers when they don't need to, because they don't trust the other code[rs]. If you have a tree-like datastructure such as a nested map, immutability lets you copy the structure by reference even when modifying the contents.

That said, the added complexity in memory management means that if you can afford good programmers, it's better that they just try to avoid this practice in the first place. So HPC folks take the mutable approach.

I guess it's hard to say if an average, middle-of-the-road programmer will ever be able to productively work on parallel systems.