Well for one thing, if you leave Defender II on, and you stand still, you start to generate a lot of threat.
When we've had non-gladiator tanks (ie, a glad doesn't show up so we let a Pugilist tank), once you get Steadfast your threat generation skyrockets and I've pulled hate just spamming Brutal Swing II (say to knock off a buffalo horn).
I always keep it off unless it's my turn to tank, or if a tank goes down for whatever reason. It takes a second to turn on, and once it's on I leave it on for the duration of tanking.
As for soloing, I generally don't bother with Defender II because I prefer offense.
Zam has the wrong description, but yg has the right one. In fact maybe I'll update that now.
Tighten your guard, increasing defense and reducing attack power. Increases enmity generation while under the effect of Steadfast.
Yg (and correct):
Edited, Jan 30th 2011 4:09am by Eadieni
Tighten your guard, increasing physical and magic defense while reducing attack power and magic potency. Increases enmity generation while under the effect of Steadfast.