As a discipline priest, reforging allows for a ton of options and the ability to customize your play-style as a healer. As an example, I will describe what reforging allows.
If I want to be an atonement-style priest, critical strike rating is an excellent stat, which allows for more damage, and higher heals through that damage. It also increases the chance for divine aegis, which is nice as well.
If I want to be a defensive-style priest, mastery is an excellent stat, increasing the size of my bubbles and aegis on my targets.
If I reach a point where my mana regen is good, reforging also allows me to adjust my spirit and throttle it as needed. This is a nice option.
With no ability to reforge, I might get 5 consecutive drops with haste, which is not a particularly good stat for me. I will be stuck with these stats, no matter their usefulness. I might be forced to play a different style according to my drops, instead of playing the style that I want to play.
In addition, other healers who have a "bad" stat, might be penalized more than those that do not. If haste, mastery, and crit are all equally good for shamans, the gear might not effect them as much as another healer who has a "bad" stat because of the design.
I think reforging brings so many neat options for a player to think about, and in the end, the PLAYER is in charge of his destiny, not the random drops that may come his way.
Just my two cents.