If DPS turn to healing then they are not DPSing. A DPS who helps with healing is less effective than a DPS who cannot heal.

You are more effective doing the role that you signed up to perform in duty. The only time you should be doing DPS and I mean, fast DPS like DOT damage not 2.80 second cast time of Holy is on trash (if the tank is not dying) plenty of time I've had a healer spam Holy while my HP is 20%. On big boss fights heals should not dps and instead should be keeping their MP pools good for when the shit hits the fan.