The point of an air gap is to make data transfers much more controlled. Some can be crossed regularly (with appropriate control), and some should not. One should only adopt any security measure after a cost-benefit analysis. The depth and rigor of that analysis should be determined by the expected costs (ongoing/operational) and potential costs (from a successful exploit).
Thus, I said "If I really wanted to reduce exposure", not "Everybody should do this to reduce exposure". If the productivity costs are very high, you had better impose enough oversight to deter or catch any policy violations... or choose a security policy besides "air gap". My basic points stand: much more software regularly talks to a network than regularly reads from CDs, and the protocols involved are much more complex for network communications; and USB sits in between those two.
FWIW, industrial control instructions can be made much more regular than arbitrary data, making it easier to detect a compromise before it reaches its ultimate target. For example, if the usual file size is 1 MB, you had better have a good reason for it to suddenly be 3 MB. If you are really paranoid, you might have a format checker or sanitizer to act like a very application-specific antivirus.