Well, I might ask, why carry 3 pot calls, or a push pin?...similar thought can apply to a longbox and a shortbox. Both boxes, as well a hen box can be very effective depending on the birds, the day, the breeding period, the weather and so forth. Presuming one knows how to run both a LB and a SB, a LB is and always will be louder than a SB, but that is NOT the reason I carry both, nor the reason I might choose to run a LB versus a SB at any given moment on the hunt. They simply sound different than each other, in a similar manner a slate sounds different than a glass pot call. It offers diversity in sound to the gobbler or to the hens and lends another tool in your call arsenal.
Agreed that a LB is not convenient to carry and there hasn't been a vest designed yet to carry one easily. IF, you really spend a lot of time running each and exploring the sounds in each you will hear distinct turkeys...variances in clucks, yelps and gobbles. There ARE more hens in a LB. A LB is not for everyone that is for sure, but I propose like any other call(s) if you have confidence in it, or any call for that matter you are more likely to carry one, run it and be successful, thus that's what your take to the turkey woods.
I also like the fact that if you fall hunt, a LB will be your fundamental GO TO call.
