I don't know if I would go as far as to say the lighter suburito are good for tanren, but I do like them for form. I think many of the exercises we are discussing are probably better with a lighter suburito, rather than the heavier version. I tend to like the heavier suburito for centering and body unification; I like the excellerated fatigue from the heavier weapon and the reliance on body structure. We have several students strong enough to cheat with a lighter weapon who get fatigued with a heavier one. We often fatigue ourselves then move to a lighter weapon for training... I just heard a story about a class who's students were instructed to hold kamae for 30 minutes...ouch. That's fatigue.
My personal opinion is that learning to move with ki/kokyu skills is hard to do because it represents (when done fully and correctly) a radical change in the way a body moves on a couple of fronts. So first, IMO, a person has to be shown how to do it with no weapon and the basics have to be ingrained. Once the basics are ingrained in moving with ki/kokyu/hara then it's time to begin to add a little weight and let the newly-developed (developing) ki and kokyu strengths be trained a bit more, then a bit more than that, and so on.
To jump too quickly to "use internal strength" on a too-heavy suburito or in "doing martial applications" is pretty much one of those steps too far, too soon, for obvious reasons. In my experience, when I've met someone who jumped too quickly into heavy suburi or "doing martial applications" the part that suffers is the development of internal strength.
If you think about it, it's the same reason why weight-lifting is admonished against... it's training that is counter-productive to that hard-to-do real change-over into real internal strength. Don't get me wrong, Jon... I'm not admonishing anything you're doing, but simply stating a point of view that I have.