ABC - Always Be Casting.
I main Holy Priest and M+ is pretty much the only thing I care about in the game and have since Nighthold. I am by now means high key pusher and all keys that I do are pugs.
I find it both fun and challenging to weave in DPS as much as I can - be it on boss pulls with Heroism where I try to use that entire 30 second window to DPS and just do that or throw out Shadow Word: Pain with Holy Fire on top of big HP adds while I Smite tanks main target. Does my DPS make or break the run? I doubt it. But it's fun and sometimes there's just nothing to heal, especially if pulling is slow and the DPS are on top of CC game (which they rarely are in PUGs).
I also have a Resto Shaman alt that I try to play as much as I can and weaving in DPS on that one is a lot more fun because of Fire Shock procs and Venthyr ability/Legendary which I chose specifically for some juicy numbers on the meters with Chain Harvest goes off.
I have tried healing in other MMO's and still heal on Classic as well and let me tell you - both versions of World of Warcraft handles healing very well. In other games, healers are DPSers with two big healing spells and some minimal utility.