Like all airlines, they're in a cash crunch right now. Before they push through a schedule change which will cancel a lot of flights and make passengers' reservations subject to refund, they'd rather give you future credit (which you may not wind up using) and hold on the money you've already paid.
Not worth it for me given their limited nonstops out of STL, their ridiculous connections, and the potential that I'll lose the credit if they go under. I'd rather take the sure thing of a refund or be able to dispute the credit card transaction if they go under.