I don't know about priests since you only have one HoT, but as a druid bigger heals does at times does mean less healing, if I can pop a triple stack life bloom and and rej, even on some raid bosses (unless there is the unexpected spike damage) I can sit back and just pop a LB once every 6 seconds, and rej every 10 seconds.  Less +healing means I have to pop on regrowths to help keep the tank up more often.

850 to 900 hps just from 3 LBs is not that bad and about 1000 every 3 seconds from Rej is also not that bad, that works out to be about 1184 hps on the tank depending on damage in.

And since (from mostly spirit) i have about 320 to 350 (depending on raid buffs) mp5 while casting (have to love Intensity http://thottbot.com/s17108 I have well over 700 spirit in raids)