Seasoning is all about relieving internal stress in the wood. When a tree wants to straighten up from wind or from starting to fall over, it makes reaction wood. In yew that is compression wood, which is made at the compression side of the trunk. It can also be found on the underside of all the branches. The reaction wood expands and contracts according to the air moisture. If a stave contains both regular wood and reaction wood, it will bend itself during drying. - that's why it is importand to leave the tips wide untill the stave has dried.
The activity of the reaction wood seems to go down over time. Read the book "Bows and arrows" by James Duff, from the early 20th century. Wood beams from old houses were preferred for arrows for this reason. Really well seasoned wood was also preferred for cabinet making because of less internal stress in the material.
The same goes for bow wood. If the wood in the bow is fighting internal stresses, it won't be able to cope with as much bend as wood where all the cells are dealing with only the stresses from the bending. Also, the wood is more predictable in that it won't bend itself during making it, or even after it is finished.
I have one yew bow which will pull itself into near a foot of reflex when the relative humidity is low, while it is close to straight when air humidity is up in the 50-60%.
Seasoning wood is easy for someone who is making a lot of bows, while it may be hard to wait if one is only going to make a few bows. It is easy to see why many willingly disregard the importance of seasoning. After all, one can make good bows in just weeks. The advantage of well seasoned wood is perhaps not that great, but it is there, and I don't think we should disregard traditional knowledge as easily as some have here...
