Two seconds may not sound like much of a wait, but it can be the difference between effortlessly healing an encounter with a feeling of control and playing catch-up by using a variety of methods.
I have come to realize that I did employ a "rotation" of sorts for Disc. It would start after combat has started and regular damage was incoming. It would go somethng like this:
1) Shield + Penance on tank (raid check). PoM and/or Renew on tank- whichever is applicable.
2) Shield (raid) + Fheal (Renew!) on raid or tank, repeat as necessary, trying to end with a shield in raid before-
3) Penance + Fheal (Gheal!) on tank (raid check). (Gheal is not an option if Penance is not Shield hasted.)
The places where I have (raid check) are times where I would choose between going to individual heals (Shield/Fheal, Shield/Fheal and so on) or dropping a Shield/PoH on a group. Obviously, if the tank is taking constant damage, the heals center on him/her, but that does not remove the possibility of centering a PoH on the tank to incorporate some raid healing along with the tank healing. (Renew!) and (Gheal!) are used situationally.
It is when I use a Shield spam that I really feel the 2 second pinch. I could normally get a group shielded and return to the tank for a topoff (Penance + Fheal (Gheal!)). If I don't have that Penance to lead off the tank heal, and the tank still has Weakened Soul, I am in a world of trouble. There just isn't enough time to get off enough heals in that small window when the tank's health is dropping. I'm thinking that Pain Suppression can be useful in this situation, but it would have to be applied when the tank has full health to be truly useful. And if there is an all-star DPS in the group who is riding the threat line, Pain Suppression has the opportunity to open up a different can of worms.