We should send backup.
First, orbiter missions are necessarily safer than lander missions and have longer lifespans. They also get less detail but what they lack in depth, they gain in breadth. Granted, a second orbiter won't add that much in breadth right now, but there are several considerations arguing for sending one anyhow.
1. It is a communications relay to Earth. This is probably sufficient justification by itself. Things happen to spacecraft, even when they're in Earth orbit, and it takes at least 18 months to send a replacement to Mars (and usually quite a bit more). We don't want to be blacked out or in intermittent contact with our surface craft for 18 months. The value of the missions on the planet justifies having redundancy above the planet.
2. Stereo imaging. When something happens on the surface, we can get two angles on it at the same time, or at nearly the same time.
3. Mission expansion. Right now, MRO has to point back at us at least part of the time to transmit at high speed. This can pull it out of position for doing science. The amount of time spent blind would be greatly reduced if there were two eyes in the sky rather than one, and it might be practical to do more orbit-changing burns, even knowing they shorten the useful life of the MRO, because its replacement is already in place. Despite the "shit happens" factor, most spacecraft don't just up and die. They run out of propellant to maintain themselves, and then have to be disposed of so as not to become navigational hazards.
4. New technology. Sensors have only gotten better, and I'm sure there were compromises made in selecting the hardware for MRO that are biting mission controllers in the ass. Those could be fixed.
I'm sure there are other reasons, but these seem quite sufficient to me.