I don't know specifically for pld, but for dps classes auto attack generally accounts for around 15-20% of their total dps. I assume it's similar for pld. That means sword oath is basically a 15-20% dps boost. Remember pld has a fast auto attack so it adds up really quick.
you can generally get clemency off while being attacked. However it also shines when a boss goes to cast something. I use it to just help out the healers a bit most of the time. Boss does an aoe, cast clemency on either the target that needs it the most or the least likely to receive the first heal. Dps misses a dodge cast it, phase changes top people off. It doesn't make us a healer but it is pretty amazing having a heal that powerful that you can cast on anyone any time.
