Nothing wrong with stacking classes. I use 2 paladins, which lets me have both Blessing of Kings and Blessing of Might, plus 2 different auras and 2 Lay On Hands.

But I don't personally like Holy Paladins for multiboxing. I'm simply not a fan of their mechanics or AOE healing. If I had to guess only in terms of popularity, I'd probably say Priest > Shaman > Druid > Paladin for multiboxers. As far as usefulness, they all have their pros and cons and for every person that says X is a horrible healer when multiboxing, you'll probably have a few people who swear they are the most awesomest uberest thing ever. So pick what you like. You can always level a replacement or swap specs among your team.