"I do some of my best healing while dead." --Grizzled, priest on my server
Priests are such targets in battlegrounds, and have such little resistance to the assist train, that you spend a lot of time kissing dirt. Spirit of Redemption greatly increases the healing you can put out. I'm kind of torn between it and Pain Suppression, since all things being equal I prefer to survive, but SoR definitely has its uses.
Spirit of Redemption has a lot of circumstances where it can be useless, so it's no panacea. It's just nifty. The 5% spirit boost is nice, but PvP gear has no spirit on it. So take it for what it's worth.
Alterac Valley gives a few places where this talent can shine-- namely, in defense of major NPCs. One of my fondest AV memories was going after Balinda while she was under fire from a Horde raid, tossing her PI, dumping a whole mana bar's worth of healing on her, dieing deliberately, and using SoR to dump 15 more seconds of healing on her. I single-handedly wiped the attack. It was delicious.