It uses it (SGS II on MIUI v4 2.4.20) but A LOT LESS than Samsung Stock or CM9 that is why we get so poor battery life on MIUI v4.
I used to get 97% deep sleep over night with Samsung ICS - XWLP7 and now with MIUI v4 I can't hardly get past 67%, it is a HUGE difference and it really affects battery life.
Hopefully as the rom matures it will get better.
Of course it depends on how much you sync. Bit with the same amount of accounts syncing I had way better battery with Stock ICS than with MIUI.