spamming Heal gave you the same healing done as spamming Smite
Speaking for myself, I'm not spamming anything. I'm using everything (except heal, ha ha). And when utilizing as many tools as a priest has, heal just doesn't stand up to the others. I find it to be terrible simply because I haven't got 2.5 seconds to spare for a small heal. If it's not instant or at least fast, then it has
to be big. They could give me heal for like 12 mana and it still wouldn't change that.
Then again, I don't Smite heal either, so I know my opinion isn't the relevant one to your question. But I would venture to guess that even in this time of "bigger health pools, more time to think," pretty much every priest can find something better to spend almost three seconds on. Even Smite is doing some damage, for according to your math the same amount of healing, and with half a second less investment. Plus significantly more mana, true, and certainly mana is a big issue right now. But heal is just so unimpressive to me that I can't think even its high efficiency is worth it.