It is nice to have the bonemass up for the ice attack it really takes the hit off it.
That is probably why it phased you so little when you did get hit.
If the fight lasts too long and you get hit while bonemass is off you will lose a good chunk of health and you have to be a lot more cautious.
That said though I will typically only have a bit left to do once bonemass ends so it works out well.