That attitude doesn't fit the game, and in my opinion, the game would stagnate if so. We were pretty good without lightwell use. I'd hazard a guess that hardly any priest took lightwell pre cataclysm. We functioned very well without lightwell. They still improved it to the point where it's the best healing hot in the game bar none.
My personal preference would have been, to be honest, if they had abandoned Lightwell. It is very mana-efficient and situationally useful (e.g., Furious Roar on Halfus), but I'm not even convinced that having DPS divide their attention between encounter mechanics and using the Lightwell is an unmitigated good. If it disappeared tomorrow, I couldn't care less.
I think in general, they made Lightwell work because they wanted to show that they could make it attractive despite general player dislike, not because it was ever the pinnacle of game design.
In general, though, they haven't really cared about a spell being very situational and underused. It just comes with the territory of classes that have a large spell repertoire. Witness, for example, this statement by Ghostcrawler about Healing Touch in WotLK:
"Likewise, we think Healing Touch is fine. You use it at low levels and you use it with Nature’s Swiftness. Maybe you use it when healing a 5-player dungeon. Druids have a lot of heals and if we made Healing Touch more usable it would come at the expense of another heal like Nourish or Regrowth. The problem again is the talents. It doesn’t make sense to have so many talents support a situational spell."
You could write a similar statement for HW:Sanctuary, just switching out spell names and the precise circumstances of its use. Like, this:
"Likewise, we think Holy Word: Sanctuary is fine. You use it when you need more throughput than other AE heals can produce. Maybe you use it when healing a 5-player dungeon. Holy priests have a lot of heals and if we made Holy Word: Sanctuary more usable it would come at the expense of another heal like Prayer of Healing or Circle of Healing."
Just to be clear, this is totally made up by me -- Ghostcrawler never said this. But it's perfectly analogous.