Currently, I use Holy Word: Salvation in some of my keys, most raids and battlegrounds. I have found that in keys if you are in need of a fail safe, you run this spell with holy mending and you stack prayer of mendings. Then use Holy Word: salvation to give your Prayer of mendings an extra oomph that could keep your team up in just about any key.
In some of my details I have seen holy mending do 2.3million direct heal and 800k over healing. Along with Empowered renew doing 2.7million direct healing and having similar over healing.
I haven’t used salvation in keys in a long time. I did as a crutch during fort weeks in halls of atonement for thrash and those dudes in de other side that did the aoe grossness. When I had enough ilvl to feel comfy I switched to apotheosis.
So, I get using it as a panic button. But, apotheosis is way better in keys imo and haven’t used salv in keys since those two dungeons when I was low ilvl.
I don’t judge peoples choices though. If it works for you in the keys you are doing, then keep at it. 
I haven’t tried using Holy Word Salvation in keys. There really isn’t enough situations I can think of where I would have to use it and at most it’d only cover one boss mechanic.
Holy Mending didn’t provide enough hps for me to pick it over Benediction because Benediction procs like crazy on some of these fights and can keep like a 80% uptime on Renews on people.
Not to hijack the thread, but is power word:salvation better for disc?
There’s no Power Word: Salvation… Holy Word: Salvation is a Holy Priest talent, not available for Disc.
Do you mean Power Word: Life perhaps?
I did. Thanks for catching that.