In "First Contact" (S5/E10) Drs. McKay & Jackson find one of Janus' secret labs on Atlantis. In the lab, there is a simple-looking box which acts as a homing beacon for a highly advanced race who quickly steals the box as well as absconds with McKay & Jackson. When they return to their icy planet, (it turns out they are cloned Asgardians that migrated to the Pegasus galaxy long ago), they force the doctors to get the "Attero device" working again now they have the missing part.
The Asgardians indicated they needed McKay's ATA gene to get it working. However, they've been living and operating on a Lantean outpost on the icy planet for nearly 10,000 years (another Janus secret lab) as well as they are flying/fighting in Lantean spaceships. They've seemed to have gotten past Lantean gene safeguard long ago. Further, Asgardian knowledge/technology is superior to humans.
Why then, would they need McKay's help getting the Attero device to work again?