Somehow I fail to recognize the need to use only the pointed end of the yari. Why do you consider I cannot thrust with the blunt end?
I don't. You're supporting my point rather than disagreeing with it, Alejandro. Either:
1)Your weapon has two distinct ends, such that only one end can be used for certain techniques -- in which case, it matters which end is which, or
2)it doesn't, for all practical purposes. If you use both ends of your yari for all techniques, then for all practical purposes, it falls into this category.
To return to the subject of this thread, the stated purpose for having a jo with different colored ends is to allow the student to visually distinguish between them. This would make sense if you were using a staff in order to learn, say, a naginata kata: you have a practice weapon that doesn't have a single "business end", and you are trying to train in a technique for a weapon that does, so you need to somehow mark which end of your practice weapon is the "blade" end. But aiki jo techniques and kata don't do this -- the practice weapon IS the real weapon, not an imperfect surrogate, and the techniques don't distinguish between the ends.