I keep a harmony on switch from a WF for 2 main reasons.
1. casting nice, high level valks
2. killing physical immunes.
Even though the Harmony I've got is on a razor bow, I couldn't care less about physical damage as it don't make a difference to PIs. I've tried waiting for amp to kick in with a WWS, but switching to Harmony for PIs is just so much quicker, AND there is always the vigor to run away when the PIs start to overwhelm.
However, I don't think I could survive anywhere else with it. If perhaps Harmony is done on a bigger, badder bow such as a Mat or a GMB or Great, it could be a good end game bow, but it certainly depends on playing style and personal preferences. I would think that speed would be better than damage on a Harmony (for my uses at least).
cheers!