Yeah, I've been milling over this glyph more and more, and I've gotten to the same conclusion as Kelerei:
1 tank fight, I'm tanking: Can't use it on myself, useless.
1 tank fight, I'm not tanking: I'm probably in my Ret spec then, so doesn't matter what I glyph in prot.
2 tank fight, taunt switch: The 30% damage redirected to me probably won't kill me.
2 tank fight, large area: The other tank is probably out of my range for most of the fight for me to put it on him.
Overall, it doesn't feel like it would be worth it. Going through the ToT bosses:
Jin-Rokh: Tankswitch, I could just use it on the other tank unglyphed.
Horridon: Other tank is out of range most of the time.
Council: Admittedly, it could help here.
Tortos: Admittedly, it could help here.
Megaera: Other tank is out of range most of the time.
Ji-Kun: Tankswitch, I could just use it on the other tank unglyphed. If a guardian needs to be tanked, then the other tank is out of range.
Durumu: One tank fight. (and with 2 tanks, it could simply be used unglyphed)
Primordius: Tankswitch, I could just use it on the other tank unglyphed. Perhaps on Heroic...
Iron Qon: One tank fight. (and with 2 tanks, it could simply be used unglyphed)
Twins: Tankswitch, I could just use it on the other tank unglyphed.
Lei Shen: Tankswitch, I could just use it on the other tank unglyphed.
So it's only Council and Tortos where it would see any real use.
For Ret however, it'll be a core Glyph when 5.4 hits. Partially because Ret doesn't really have any decent glyphs.
I think that's looking a bit too narrowly at "other tank".
Jin'rokh means that you can use it on "that guy" who keeps finding ways to just kill himself during Lightning Storm. Council allows for use on Frostbite as well, or a sketchy Discharge. On Megaera, you may not necessarily be able to use it on the other tank, but it could be invaluable on the guy who just gets Torrent right before Rampage is about to start. Durumu adds extra cheese for Parasites, not that they're particularly hard, but it might be useful anyway. Primordius on Pathogen. For Lei Shen, on things like transition Shocks, or people with Diffusion in the final part, or people with Helm. On Ra-den, on the guy who is out when the reset is going to happen. etc.