1. Although the Athosians are known as traders through the Pegasus galaxy, what they trade predominantly isn't goods, but rather introductions. An Athosian with a trading party is as good as a guarantee that you won't get cheated or sold out to Wraith worshipers or the like. (It isn't until well into the expedition's second year that John realizes how lucky they'd been; those early months hadn't been easy, but without Teyla and the other Athosian guides, things would have been downright hairy.)
2. They have vocabulary to describe pretty nearly every possible relationship, blood or otherwise; second cousin's mother-in-law? Something that sounds like a sneeze. (Teyla collapses laughing the first time John attempts it, and he can't really blame her.)
3. Halling can give his family history going back eighteen generations; Teyla knows fifteen; some of the older Athosians can give details as far back as thirty, although those are pretty spotty. And between everybody, they can give rough histories for almost fifty other cultures.
4. This is partly because most of the Athosians are actually refugees or the children (great great great grandchildren, in some cases) of such. Not that their home cultures had been destroyed (well, not all), but if your town's been culled and someone comes along offering your family someplace new to start over, why not? (And this is one reason why everyone knows and trusts the Athosians, to the extent that many children in villages near stargates are told to go looking for the Athosians should their homes be destroyed.)
5. They still tell the tales of great battles fought by the Ancestors against the Wraith, of their heroes and sacrifices and promises to return. And some still believe that the expedition came in fulfillment of those promises; they accept John's protests to the contrary with smiles that clearly express that they are merely indulging him.