Those 5 seconds are used when the excrement hits the fan. One does no go around merrily activating Resilience all over the place. You keep it available so you can buy time and live in critical situations.
That maybe true in the lower Ops and Flashpoints but in more demanding Ops (Denova NM atm) you have to use Resiliance not only as "oh ****" button, but use it to catch the huge Force /Tech attacks. Like the Dot from Kephess or the Jump from Z+T or in the Firebrand and Stormcaller encounter when you planted the double Destruction debuff to your mates and the healer is keeping them up and cannot heal only the tank.
In General being a good Shadow Tank (which is the only tanking class i can really speak about) is about knowing when and which damage is incoming. For example using Resilience during defensive phase at Firebrand and Stormcaller is just silly because it will do nothing for you. Using your defense Cooldowns proactivly to keep you at a health sweetspot (100% health is a waste of selfheal) is a big part in Shadow Tanking and stealthing when the excrements have allready hit the fan is just saving you a lot of Rep costs.