why can you not enable bottom halves when irqs are disabled?

Question

In __local_bh_enable, we have a

WARN_ON_ONCE(!irqs_disabled());

I'm wondering what the purpose of this warning is -- I would imagine you would want to be able to renable bottomhalves when in say spinlock_irq protected code... (note: I'm looking at linux 3.4 code here, but it also seems to be in 4.1).