I think the why its desirable is that without the bladder, the big bass reeds have trouble reliably sounding – if I open the valve THEN blow, its difficult to get the reed to sound with good attack. If I pressurize the chest THEN open the valve, the reed sounds reliably.
I think that is indicative of one of two things.
– a charged bladder is necessary for reliable sounding because of initial puff volume
– a discharged bladder will stop reliable sounding because it lowers the speed of pressure rise
But if we combine that with the other data point that I’ve heard (though not personally experimented with)
– these bass melodicas don’t work well without bladders
it would tend to cancel out the second option leaving only the first.