Priest Holy Nova causes 0 threat - both when used offensively and as a heal. So when you're AOE pulling a mage and a priest on follow, you can put out a bit more dps and get some healing at the same time with 0 risk to your priest. Also PW:Shield does its aggro when cast - not when the bubble actually absorbs damage. It can be weird seeing your priest surrounded my mobs that they're killing, but still not attacking them. I found that helpful a few times when doing some open world and ZF zombie farms.

I actually realized that I wanted another priest after I'd already leveled my 1st mage solo, and then my 5-box team separate. I found it was quicker to level a Mage+Priest then it was to level Priest solo. But that still wasn't as fast as leveling a solo Mage.