It isn't so much that some brokers will require air ride suspension... it is that the customer requires air ride. fragile freight will incur less damage with air ride. Who cares about paper towels or blocks of lead. But sensitive electronics like computer components or appliances are another thing.
Seriously, I wouldn't pull a spring now a days.. You'll get better service, lower freight claims, and find more loads w/ air.. I've had a bunch of broken springs over the years but never had trouble with an air ride..