Quote from Maverick74:
Haircuts are calculated daily. So yes, like span, they change as the underlying moves up and down and as times goes by. The problem with span is that the margin levels themselves can change and quite dramatically around volatile events or volatile markets. They can double or triple overnight. So if you use span too aggressively, you could be in a situation where you have no control over your position.
Thats true. Span is calculated based on the probability (or volatility). At "panic" time, volatility goes up, and so is the margin. You are likely to be forced to cover if you don't have enough cushion for the margin change. Using haircut, it is a lot safer.

...much better with a partner