Brand support is basically a bot lane mage who's only use is damage, of course he's going to be dealing damage, he's not a "support" (inb4: HE HAS A STUN!).
Sona: Jack-of-all trades support, she has decent peel, escape, engage, CC, and healing but not great.
Taric: I can't really argue... Not smart enough
Nami and Lulu: both of them are very aggressive supports, probably some of the most aggressive (that aren't bot lane mages like Brand, Annie, Vel'koz, etc) along with Morg; they're very peel heavy.
And at the end of the day, true supports aren't supposed to be dealing the damage, they're supposed to be supporting the damage sources (Even Taric will be doing less damage than the carries). Janna already does a fine job of protecting without needing the poke damage. And if she does get some damage buffed, her utility will need to be toned down.