I would like to point out that the real benefit of Serenity isn't so much the slightly better single target healing, it's the rolling Renew mechanic. An example would be hardmode Conclave of the Wind, when you are stranded on blue with the tank. It's easy to keep Renew rolling on both of you which you can keep up with a mana neutral Heal or Serenity cast. If you both take enough damage, a quick Binding Heal will refresh both Renews. By all means, you could just sit in Sanctuary and throw out the same heals, but I prefer to save my mana for when I actually need it.
It's still very situational though. I find Sanctuary to be better most of the time like some of the other posts here, but if Serenity is a good fit for a particular phase you might as well use it.