I guess you're kind of getting in to realm of performance annotations.
I think there are still quite a few open questions around it, but it's important for some kinds of programs to commit to particular performance characteristics. I believe it's still something we all want to happen in some form.
I don't know if we could quantify it - to give you something in the middle between "any allocations" (the default) and @_noAllocations, but it's an interesting thing to consider.