I'll Sword Oath bosses in dungeons, but generally will leave Defiance on unless I'm with an Embrace force-spamming SCH. The DPS difference between no Defiance and Defiance isn't high enough to make your healer waste multiple extra GCDs of healing when that can be precious healer DPS, and isn't worth losing the comfort zone IMO. No Defiance WAR isn't particularly tanky without CDs, and WAR needs most of the effective ones (Conv, Vengeance) for upcoming big pulls more so than to just blow for a little extra no Defiance time. Usually if I do drop Defiance on a boss it's within the last 20% or so when no more healing will be needed anyway.
If a healer's not DPSing though, Defiance is going off as soon as aggro is established. Gonna put the healer to work if they're being lazy.