If we go back to the original poster and the context he's asking for advice. He's clearly not progressing Heroics and in light of that, I don't see how you can even begin to discuss whether or not Disc is a viable healer. For Normal mode all healers will do. Not viable to me, is what Resto Shamans were in 10 ToT Heroic. Yet we used a Resto Shaman when having to 3 heal and yes, we cleared 13/13 Heroic months before 5.4 hit.
This whole notion that some classes are so utter crap that you can't use them at all, just isn't true. "You can't tank without a Paladin tank" or "You can't raid Heroics with a Resto Shaman" - well you can, if your players are good enough. "Disc aren't viable" is BS, ofc they are. Some classes will always be stronger than others but that's not the same thing as "Not viable at all" as you make it out to sound.
I'd bring a skilled Disc over a mediocre Resto Druid any day and again - to answer the OP's original question - every healing class is viable if played correct.