Syphon Strike + Delirium is higher damage than Spinning Slash + Power Slash

Dark Mind is useful on:
(probably) some of Bismarck's weather effects
Ravana's adds
Drainage (the tether during tornado phase) and everything but his autos and conal attack on Living Liquid
All damage in Alexander 4 (except adds) - especially Perpetual Ray.

In savage, it's all of that plus a tank buster in A1S.