Hey wait, wasn't there a thread on this very subject in e-budo a while ago? Here it is:
tsuba, or not tsuba?
My opinion? Some people may regard bokuto as disposable, and therefore do not warrant embellishments such as tsuba. However, given the costs of plastic tsuba and those white rubber tsubadome these days, that isn't a highly convincing argument. A tsuba is a very nice thing to have when your training partner, inadvertently or not, slides a jo down your bokuto. Mistakes do occur during training, so a minimal amount of protection, at very little cost, is better than none at all. Hope this helps.