Aight, just my experience so far:

As a tank (Pld main):
- Depending on the dungeon / raid you are in, quelling strikes can be incredibly usefull
-> [example] "old" T7 Addspawn at 80/35% -> incredibly helpfull if the dps have it / use it
-> dungeons: large pulls -> triplle flare -> incredibly helpfull

As a Blm(left side crafted I110, full melded, right side Sol/HA I110 + Shiva Weapon I115] - NO QUELLING STRIKES
- I'm getting hate after 2 Flares allready from time to time, depending on the tank
- Attacking Adds in T9 to quickly is a death sentence

Just as example. Yes, im working my way up to Lvl 34 to get it.
Conclusion: IMHO its very very usefull if you know your Dps-Class and or raid something which includes adds spawning.