If that towel bar was heated it should be bonded, as they are, via the EGC of the circuit likely to energize it (the 3d prong on the plug). The same should be true of any plumbing going to an "insta'hot".
I certainly agree with those who say you don't need to bond every scrap of isolated pipe or other metal. Only those likely to be energized by attached equipment.
If we get back to the beam, I would only be oncerned if it had equipment hanging from it and I would want it bonded to that equipment's EGC. Incidental contact with cables running nearby should be handled with the securing and physical damage articles.
