I can keep them going 50% of the time - at rank 2 they have a 15 second duration (how long a normal fight takes?)/30 second shared cooldown that starts when they fire off. There is a per power recharge of 90 seconds, so, when that 3rd Active Defense finishes it's shared cooldown, the 1st one is ready to go again.
For long fights I've got DoT stacking/debuffing, too.
It's an interesting idea due to the Active Offenses can be used while in and to break out of holds/roots/'disable' effects, they have nice effects, damage boost, and, they take no energy which is another decent bonus for this build. 1 of them even gives energy back. 1 of them raises ego & crit chance way up, which would be nice for the heal maintain I took as well as the overall dps boost (criting conflags??), and for mega hold resist when combined with an Ego secondary + the Con advantage for Hold resist.
I might try it out just for grins.
Until I hit near 40 and can get all 3 Active Offenses, I've thought about picking up Concentration just as something to use in the meanwhile.