I'm guessing the people spending B4B are doing so because its only an 80 second cooldown and will come back up while hummel is being killed. Meanwhile if they waited they'll only get one. It seems to be a case of do you want to pad your numbers for a better parse (popping B4B early while there's multiple targets and getting it back non-optimally later) or do you need to maximize your hummelfaust damage to ensure a clear (lining it up better with other cooldowns for a pseudo-opener on hummel).
