yes its situational use and should be kept in the game. Trash for example that feral kitty that goes swipe ape shit will need one to stay alive.
Was defiantly using mine on ragnaros some times tanking with a feral tank. But then sometimes threat was buggy rag was focused on him but i was getting the stacks, no idea why.
I do like the idea of it transferring threat to the target.. similar to that of tricks of the trade somehow that would potentially give rets some more raid utility and viability in a raiding scenario. I mean they gave warriors a raid wall whilst taking rets 2 possibilites out in wrath (AM spec or amazing ulduar DG spec). Only think we have is 3% raid dmg, but you can bring an arc mage who brings more dps and time warp as well.
the threat transfer that you spoke about at the beginning of your post is what I think Blizzard is trying to avoid, this would make it seem too much like Misdirection and Tricks of the Trade, and as we all know, Blizz has been trying to avoid homogenization between classes.
I actually use Hand of Salvation on myself more than I do on DPS. I use it during tank swaps on fights like Ragnaros for when I am doing increased damage than my offtank and want to continue my rotation without fear of pulling threat back off of him.
I don't know about post-threat change, but I would usually have to use it on myself periodically when I blew my cooldowns as sometimes I would pull.
Now, I just use it on our boomkin when she gets a string of crits that turns her into an owl tank.