As far as I know, bendy buses have about twice the capacity as double deckers, despite not having an upper deck. The reason? double deckers waste a lot of internal space with stairs and (for safety reasons) do not allow standing on the upper deck).
Therefore, for the same running costs twice the number of passengers can be conveyed.
One other advantage bendy buses have over double deckers is boarding and alighting time, since they have three very wide doors, unlike double deckers where a lot of time is wasted waiting for people to descend staircases (and the inevitable blockage of people that occurs at the foot of the stairs).
My first job after uni was at a design consultancy studying the ergonomics and design of the new London double decker and boarding and alighting times with OPO (one person operation) replacing Routemasters.
The most significant factor in timing was often the ticket transaction with the stationary driver, not alighting, so door width/number would usually not make a lot of difference.
Of course it would when there are many more people alighting than boarding eg at interchanges but on the whole it is boarding times that define stop times.