I've never learned to use a bo staff (or any other weapons). I remember a Kenpo Teacher telling me one time, though, that some people use a "tapered" bo for forms in competition, because they can move them faster. He told me that these tapered bos are not as strong as the "real" ones.
If I were to be taught the bo, I would prefer to always practice with a "real" one -- that is, one that would not break. You see, I would like to be able to pick up any staff and use it.
I believe that if one were to practice with a heavier bo, then one could use a lighter one easily. However, if someone practices with a light bo, perhaps they would be at a loss to use the heavier one. This adds nothing to the thread, of course, because I know nothing about the staff. I appreciate the comments regarding which bo to use (a "log"! --> hahahaha! that was a good one! --> I can imagine someone trying to use a telephone pole!)
I have and use a tapered bo for "open" weapons in competition. It is 3/4" in the middle tapered to 1/2" at each end and is very light. It gives me the ability spin and throw the weapon in my creative forms. I do the majority of my training with my heavy bo, since the forms are really made for it. The use of each style of bo has its purpose and place. To not explore other types of use would be to limit my training and knowledge.