It should be added, it also depends on your discipline build.
If you're "tank healing disc", you have Penance glyphed and the 4-pc is a very good benefit.
If you're "raid healing disc", your Penance is not glyphed, and so does not have full uptime (on top of which you may not even be using Penance on cooldown at 12 seconds, unless only to have ten of added spirit regen). In that case, it's not overly worth it.
It depends all on your build and/or role in raids on whether this set bonus is worthwhile to you or not.