Is industry experience necessary for a patent agent/attorney? Or is just useful for getting that first job?
Not necessary; not necessarily even useful.
True. But it's also true that industrial experience can be very helpful to getting a job and to doing a good job as a patent practitioner. If you are work for a few years in industry, you will have a better knowledge of the products, processes, vocabulary, competitive pressures, and players in the field of art. But even if your first patent job focuses on a different field of art, the general background can be helpful.
I've seen too many patent practitioners, especially attorneys, who may have an adequate grasp of the law and patent procedures, but are clueless when it comes to how things actually work in business or industry. A few years in industry can provide a better understanding of the value (or lack of value) of intellectual property. Too many attorneys see their job as to minimize risk for a corporation, not understanding that they, like everyone else on the payroll, should be trying to balance risk with reward.
I've seen corporations pursue patent application to appeals, RCEs, etc. when the technology and business environment already made the application irrelevant.
I've seen corporate lawyers prepare a non-disclosure agreement (NDA) to be signed by fly-by-night small business, who doesn't respect the NDA. The corporate lawyer thought that the NDA somehow protected the corporation, but the right approach would have been to walk away from the small business and work with an established firm instead.
I've seen attorneys negotiate convoluted contracts, when the right approach would have been "Here's our first and final offer. Take it leave or leave it."
Robert K S is correct, industrial experience is not necessary, not necessarily even useful. But it can help the attorney / agent to do a better overall job for their clients.