I don't train with a tsuba, and in my fairly limited experience (only my own dojo, and visiting two others) I've never seen a tsuba on a bokken. I don't think I've even seen them in videos. O Sensei and his students didn't use them, as far as I can tell (http://www.youtube.com/watch?v=i27obVRzIPc)
. My teacher's teacher, Morihiro Saito Sensei, didn't use tsuba (http://www.youtube.com/watch?v=zQqHlyPrD6o)
. We don't have them on any bokken at our dojo. So I don't use one.
"The Last Samurai" is hardly a historical document, but no tsuba there, either (http://www.youtube.com/watch?v=g2lMcgvixN0)
. And for what it's worth, none here, either (no idea what moving this is, or if it's historically correct): http://www.youtube.com/watch?v=2jW8t5jZyEk
My understanding is that bokken is a weapon in its own right, a blunt hitting instrument (albeit with very concentrated force along the "blade" edge), not a practice sword, although it can be used for training sword technique, too. If you're practicing sword techniques, I guess you should use a tsuba, since real swords have them.
I suppose my answer is very simple: The art/lineage in which I train doesn't use tsuba, so I don't use one. If what you are training in uses them, you should use one. Why is this a pro/con debate?