Did some data collection with a friend.
Lv60 BRD with AST in party can maintain Foe Requiem for 66-69 seconds.
With basic Ewer somewhen in the middle, this increases to 96-99 seconds, for a gain of 27-30 seconds. For now, let's assume that the 3s range is a side effect of the timing of Foe Requiem activation related to the server tick, and that Ewer very likely bought is 30s in all cases.
Then consider a few of the DPS applications.
Balance increases one person's damage by 20% for 30s.
Ewer increases party magic damage by 11% for 30s.
Extended Balance increases one person's damage by 20% for 60s
Extended Ewer increases party magic damage by 11% for 60s.
Expanded Balance increases party damage by 10% for 30s.
Base Ewer increases party magic damage by 11% for 30s.
So in a magic heavy party, it might even be better to throw a base Ewer at the BRD instead of burning it and maybe ending up with something undesirable like an Expanded Bole. I don't have time to do the maths right now to say where on the good idea / bad idea spectrum this falls for serious full party duties, but certainly if you end up in a caster heavy full party roulette or alliance raid, it bears consideration.