I would definately fix this. If the other bolt fails, your mainbeam will break, the mast come down and you are left floating with two partially connected hulls. The weather will probably be foul as well as this puts the largest load on the boat. Your call, but I would want this corrected on any boat I sailed.

To fix this, you will need access to the inside of the beam. You will probably have to disassemble the boat and remove the compression pads inside the beam to get access to the inside reinforcments and the bolts. Have a look at how the dolphin striker is tensioned. There is probably some mechanisme (nut/bolt) where you can take some of the pre-tension in the beam off.
The compression pads might be glued in place, or have a layer of corrosion blocking material on them. A 2x4 and a slegdehammer might be neccesary to remove them. Once removed, a spanner taped to a batten is the proper tool for holding the bolt head while the nut is removed.

When you disassemble the boat, you will probably find that the strap is secured with additional flathead bolts. Take them out and inspect the whole load, or replace them with new bolts of the same hardness.

Half the job is starting, as Marcus Aurelius said. Begin by having a look inside the beam with a flashlight.