Well, 2.5m damage may not be as big in 25man as it is in 10man, and in 10 man you would never use DS for this fight, cause the early whirlings, when you easily can stack up, are easily doable without. If you were to pick a talent for pure healing output when it's needed, cascade is the way to go for garrosh 10m.
There is also another way to look at it, when you start to progress a fight, people will get better and better over time, that is how it works, if i as a healer could help out with a large portion of damage where its really needed, it might take us 10-20 less pulls to get confidence with that phase.
Dps check is really tough also, wich is the main reason a few of the first 10 man kills were made with one healer only.