Why would you ever want sustains active outside combat in DAO? I had them set up in Tactics to activate only when my mana/stamina had dropped low enough that the sustains didn't cost anything.
Some builds don't mind the cost of sustainable abilities plus you can make quality willpower builds. The problem is that if they exist and are turned off everytime you drop out of combat then it's the most annoying thing.
For example my autoattacking rogue build in DAO used momentum and the stun ability only. I never needed to turn momentum off.
My 2h burst warrior in DA2 (which turned out to be pretty crap but anyway) used a quality build that had enough stamina to use all the low cooldown skills and could use second wind for the rest if needed.
Blood mage is the other obvious example that can go negative mana without a problem.
On the other hand I think toggles are a bad design and exploitable. Blood magic that changes a function, or some active ability like Wrath of the Elvhen and Bodyguard are fine, but more that just add modifiers (like elemental aegis, turn the blade, control, might, rock armor, etc) are just idiotic. They can instead make them passive and increase the cost of the active skills. Fatigue was a pretty good idea, but they can do without.