Not much to add to what's already been said really. In my experience though, many pen makers think that being postable is essential, and consequently they won't buy a kit if it isn't. However, their convictions are not always necessarily based on evidence, since many of them have never tried selling a non-postable pen in order to find out.
Balance is a consideration, as has been previously mentioned, and a posted cap can quite easily make a pen top heavy. In truth though, you'll probably never please everyone, no matter which way you go - there will always be someone who won't buy your pen because it isn't postable, and there will always be someone who won't buy your pen because it is postable.
If you want statistics, Beaufort Ink sells more non-postable kits than postable ones. There again, we produce more non-postable kits than postable ones, so all you have to do is work out why we produce more non-postable kits than postable ones. 
