Barandis: So your priest is very good and carried you, gj.
Its simple, if you end the fight without expending your mana, it means you didnt use reju. If you descide to avoid using reju to keep your mana, you will be healing for alot less.
Edit: Just as an additional note, losing 2000k of Spirit will only give you enough Int or Mastery to increase your healing by ~3-4%. And you will reduce your mana pool for that fight for ~8% or more.
You're looking at this using the extreme ends of the spectrum when there is a happy medium. It's possible to both use your Rejuv and end the fight at zero mana. I proved this last night when our Disc died with a minute left in the fight, leaving only me to heal the remainder. Lol
If you look at the log, my top spell was Rejuv and while I did a lot less in hps (stupid bubbles) I still did significantly more in healing. Trust me, I didn't do all of that in the last minute of the fight because I was fighting to heal people with a sliver of mana and everyone was using personal cds to stay alive. (I did this with 7237 Spirit btw, if that matters.)
Also, when you said mana pool, I'm not sure if you were referring to the size of our mana bar or the total amount of mana we will regen over the course of the fight, but our mana bar is a fixed size now. I think you understand that, but just wanted to make sure.
Edit: It's also worth noting that on our 1st Elegon attempt last night, which lasted 9 minutes 40 seconds before we wiped, the Disc couldn't see the debuff on his raid frames, so I dispelled 14 times vs his 2. Dispelling is a mana sucking task, but Rejuv was still my top heal during the fight and I was low on mana right before we wiped, but I don't recall being oom yet. I didn't do as much healing as the Disc, but my point is that yes you can use Rejuv a lot and still be ok if you manage it well.