Please do the maths first and ask questions after. Grief+Beast does more damage over TIME due to the added damage of Beast and also the AR. But why not dual griefs? Most BvC use triwhirls anyways, making the first hit count even more (and that's where Beast has an effect on). If I right now had the time, I would volunteer to do the maths, but I do not.
Also, Beast adds str, allowing more gear changes.
To put it simple: Beast adds damage to Grief. Beast also adds OW and CB. This makes Beast+Grief do more damage over time. Where Beast cannot cover the loss of damage in OW or CB, it covers it by enchancing Grief's damage.