I've totally been wondering this myself.
I sometimes train with a couple of friends in our own little dojo, and the three of us have very different styles.
Ninijitsu,
KungFu and
Aikido.
Good fun, but I wouldn't go near the Ninjitsu chap without a tsuba attached... Its such a violent art and with knuckles exposed they seem to become a target for students in that art.
I use them whenever I can, even in class: but it does make bokken that much more difficult to carry around...