When I started with Saotome Sensei nobody used tsuba, I think primarily because there weren't bokken available with them here (I don't count the little plasticthings). Saotome Sensei was able to get Mr. Kiyota of the Kiyota Company to produce some to his speicifications with tsuba and the started to catch on. I haven't used a bokken without one since I did some classical sword with Ellis Amdur Sensei years ago. The tsuba allows you to do some things with the sword that you can't do without one. Almost everyone at my dojo uses one at this point. This is also helped by the fact that two of my senior students are also students of Kenjutsu under Phil Relnick Sensei so they also prefer the tsuba.