It suffers from a few key problems in raid. First of all the mana cost, on long fights in raids it's a ton of extra mana to dump where just a static beacon doesn't cost anything. The other factor is that the targets chosen are random and you could get people that are at 80% HP vs 30% HP. And the last one is that by the time you Virtue, then get 1-2 casts off it's very likely that at least 1 if not 2 or more of those people have already been topped off by your other healers.
Beacon of the Lightbringer makes our Mastery a lot better, because it basically guarantees people will always be within 20 yards of either you or the tanks, while also buffing Light of Dawn by 30%, and Light of Dawn does a pretty significant amount of healing in itself.
Also:
https://www.warcraftlogs.com/reports/JRPmQ7x386DZkGTv
Logs for our legendary shoulders if anyone was wondering.