Regrowth is only bad if you're overhealing. The mana per point heal is pretty equivalent to HT/rejuv (at least when I compared the values a couple levels back). Additionally, having two regen effects on the tank gives you more time to regen mana before breaking the 5 second rule again. Your tank shouldn't be lower level than the mobs. They could have excellent equipment, but damage mitigation is crippled vs higher level mobs.