Well, that begs the question, if you don't want to compare against MP5, what do you want to compare against?

Trying to quantify healing output is somewhat impossible, since you have to model the boss and all the players in order to give that a meaningful number. For instance, your healing on Heigan is not going to be the same as your healing on Patchwerk, which is not going to be the same as the healing you do on Maexxna. They're very different fights that require very different types of healing. It is tempting to try to treat healing as reverse-DPS and compute some numbers for HPS, but healing depends entirely on the fight, the gear, and the skill of your group. MP5 is a stat that is about as insensitive to all these issues as you can get. MP5 is always MP5, and more MP5 is useful because it allows you to throw more heals... however gearing and gemming for healing is an ever-changing process that varies from healer to healer. Do you notice you need to get your heals off faster? Maybe you need more haste, maybe you need to cast more HoTs, maybe you aren't anticipating enough (pre-casting party heals on Loatheb?).

As nice as it would be to treat healing as reverse DPS, it really is impractical and depends entirely on the individuals you're healing and the encounter mechanics. Unfortunately in order to theorycraft, in order for it to mean anything, you need to have some kind of neutral rating or stat that you can compare things against... and MP5 is pretty neutral.