I don't fully agree with this. Although Penance is the most mana efficient heal we've got, I mostly use it for when things go wrong, heavy damage is incoming (ofc, I'll use it when it lands) or when someone is targeted by some abi that needs quick response. Other than that, I try to keep Penance off CD, for these situation. This might not be ideal for you HPS, nor mana effiency (as I use FH instead, to fill those health bars), but it saves lifes on those occasions that Penance is used as an emergency-burst-heal.
You're right here, although you're there to keep everyone topped (thus a bit of steady healing) and preventing them from getting damage in your Disc role. You're not one to pump up your HPS in recount.
If you got any additional questions, I'd be happy to answers as good as I can. Although, I'm not that active anymore - but I still got over 5 years experience with the priest class as a healer