In real life, I personally have a really hard time seeing my arrows mid-flight (mind you, I use a compound bow now, long bows shoot them much slower and can see them better), but why should we see them in games? I mean, yes, it would definitely be nice to see them in this game, especially since (last I checked) the sights are still a bit screwed up, and it's not like the devs are aiming for realism in any sense of the word, but at this point, I'm fine with arrows being invisible or visible mid-flight. But I'd rather they fix the sights on the bows.
Also, I imagine projectile traces would consume more PC resources... which this game already struggles with. So, I don't know if it's worth it.