You never produce a live event like that without spares, everything can and will break.

Your idea about the conference buying spares for everyone is interesting. Unfortunately for one, the financial incentives are just not setup in that way. Generally each booth is responsible for its own gear. Second, everyone needs different sizes, resolutions, mounts, color profiles, refresh rates, input types, network control options, blackout behavior, etc, etc.

Also, the discount vs extra expense of the gear is correlated to the cost of the gear. A $100,000 projector (mid-range) will obviously costly less to rent for three days, and is well worth it. With a TV though, they rental company is able to charge a premium because their customers simply will not purchase that type of equipment for all of the reasons above.