Imagine a heavy tank healing fight where your tank is being nearly one-shot every 10-15 seconds (or 20, or 30). Saving Grace would be very powerful in its ability to respond quickly to that damage, and only maintain 1 stack of the -10% healing debuff on your other heals - with it dropping off each time inbetween.
Alternately and perhaps more accurate of modern fights - imagine a fight where random raid members take massive burst spikes every 10 or 20 or 30 seconds - but the fight also has passive raid damage. Saving Grace again allows a healer to instantly respond with a big healing spike to the burst damage, while diminishing their ability to respond to the AoE pressure damage - which can be assigned to a different healer (ie. resto druid).
So for example - typically Shamans are good burst spike healers due to their mastery - so in an Rshaman + Priest 10m guild - the Rshaman can do the burst spike heals, and the Priest can handle the raid pressure damage and/or tank heals. The next week the Shaman can't make it, so the Boomkin goes Resto for the week - now they have to do the same fight - but this time the Priest can take Saving Grace and focus on the burst spike healing, and the Rdruid can go HAM on the Raid HoTs
I like Saving Grace in theory - good talents are supposed to be situational advantages like this - that allow greater flexibility or alternate roles without radically different power advantages - and I think Saving Grace could fulfill that just fine