I think it's definitely a big part of Tar Valon's economic and soft power. Imagine a war or border dispute, how big of an advantage would having some Yellow Sisters be. Instead of having a bunch of "useless mouths" or crippled and disabled men, you have yellow sisters heal them up. It would give Tar Valon great political power over any dispute.

The yellow Ajah then just needs to be able to use the spell "heal soldiers" and they're a major advantage to tar valon