Aggra?

I have to admit, i was kind of a Thrall/Jaina shipper.  Not hugely, so it's not like i'm angry about the fact that he's married to someone else so much as wondering where she even came from.  Even after leveling a Goblin warrior, i don't have the slightest idea who Aggra is, i guess that i didn't find her to be particularly memorable.  At first i thought she was the chick from the Twilight Highlands--Garona?  apparently she has her own vivid background and is half Draenei--and was wondering how that even happened.  I guess you have to read the book to know the entire story, which i'm not against, since i'm already a Christie Golden fan from back in the Voyager days, but i have no access to said novel without buying it off Amazon.

I am looking forward to seeing more of Thrall, but i'm not sure that i understand.  Why can't he be a leader and a shaman at the same time?