Frankly, I always hated those things. The problem was that people would be less careful (because of the perceived "softness"), and you actually end up getting whacked a lot more often and more painfully than with a bokuto.
Actually, that was precisely why I like them. Folks are scared of the bokken... scared of being hit and scared of hurting someone else. So folks too often don't really commit. They are putting the breaks on before they ever reached full commitment. What I liek about shinai is that the vast majority of folks are willing to actually hity you with the shinai while they won't with a bokken. So, especially when doing wepons taking but even in your paired forms, you get a much cleaner practice because people aren't mucking up the energy by pulling their attacks. I figure its my problem if I get hit... at least my partner is willing to really try with a shinai.