Why no tsuba?
As some one who studies Iai (British Kendo Association) to compliment my Aikido (United Kingdom Aikikai) I have found that the use of tsuba is of personal choice unless it is stipulated by the Sensei.
There is a technique in aiki-ken called Maki-otosh, this is a deflection of one's opponents weapon (be it bokken or jo) this action quite often extends right up to where the tsuba would be on a bokken (there or not) however, the jo as we know does not carry such hand protection, therefore one has to be considerate of our partner's welfare regardless.
I must confess, during 14 years of study (aikido) I rearly see a tsuba fitted to a bokken, similarly many of the Japanese Sensei I have been fortunate to see, likewise omit the tsuba. Interesting question.