I have tried hitting myself with very low powered shots and with no armor a normal arrow does 0.5 hearts of damage. The same is true with healing tipped arrows and they do not heal you at all even with healing 2. The harming ones also do 0.5 hearts with harming 2. This means they make absolutely no difference at all or it's so small that it isn't displayed on the hearts. Either way it needs to be fixed so harming does a bit more damage and health should do less damage or maybe even heal more than they damage.
This is still an issue as of 1.9.2.