Most modern FP languages (and quite a few non-FP languages these days as well) do immutable by default, with carefully controlled explicit mutability where necessary. It's basically the flip of older traditional languages where unless specifically stated otherwise everything was mutable. Doing immutable by default encourages you to use mutability only where you really need it. Immutable references allow all kinds of desirable compiler optimizations to be implemented which leads to better performing code with fewer bugs, but it doesn't need to be an all or nothing, you can have both mutable and immutable references in the same code, it just means the mutable parts don't benefit from the same optimizations the immutable parts do.
