How does Drogo benefit from the deal with Viserys? Is Daenerys really that valuable as a wife, if the Targaryens’ status is so uncertain?

The one thing the Dothraki fear are dragons, the one thing that held them back from their manifest destiny for thousands of years was the white-haired, purple-eyed dragonriders. 

It’s not about power politics for Drogo, it’s about symbolism.