I'm pretty sure the taper on the kits is to form a good tight seal. With these kits, if the end of he reed kit opens up, the call can lock up or not play at all. So the taper prevents the bottom of the reed kit from expanding, The O ring method is better for a number of reasons.
1) the O rings hold the bottom of the kit together so you dont have any chance of it opening up
2) sooner or later, a customer is going to force that mouth piece in the barrel so hard it is going to crack the barrel.
3) The natural expansion and contraction of the wood can cause the mouth piece to either fall out or get stuck so tight it cracks the barrel.
Jerry Latta of Hawks Feather Custom Calls - a member here - came up with the idea of turning down the inserts to a straight bore. I really think if you are going to use kits and not make your own mouth pieces, this is the single best thing that ever happened to crow and elk calls.
Al @ THO