Purpose of 'bore' on spoke nipples - why are spoke nipples not threaded through their entire length? I noticed that (most) spoke nipples have a thread which does not run through the entire length of the nipple. As can be seen in the image below some of the length of the nipple has no threads (and is called a "bore" apparently).



This left me with some questions:


What is the purpose of the bore? 
Wouldn't the nipple be stronger with longer threads? Are there disadvantages to using longer threads (/sacrificing some of the length of the bore to add more thread length)?
Is the amount of thread the same on longer/shorter nipples and is the difference between these different lengths of nipples just in the length of the bore? If so (if thread length is the same)...what is the advantage of using longer nipples?
Are there spoke nipples where the thread does have a longer length (thread across the entire length of the nipple)?


Thank you for any input, it's greatly appreciated :)
Adding threads to any sort of bolt under tension doesn't actually make it much stronger, beyond the first ≈5 threads. Adding more basically just adds dead mass; either way the whole thing will generally fail somewhere close to the first thread. So, filling the bore with threads wouldn't have any benefit. What it would do however is move the expected point of failure out of the relative safety of the unthreaded bore, which particularly protects the spoke from being bent at that point, which would further diminish its strength. The bore makes it so that if there's some bending, it will be on the unthreaded part of the spoke, which is stronger to begin with, so not as worrisome.