As for other issues to consider, removing a ceiling to expose the beams exposes the utility runs that the dropped ceiling might conceal or contain: electrical, plumbing, cable or phone. Removing it might make the electrical work no longer meet code. For instance, they could have used sheathed electrical cable wire for the electrical runs, which is Code if covered by drywall or plaster, but is not Code for exposed runs. You would need to re-route the wires or have them re-run with THHN wire through metal conduit. I did it, myself (re-routing some runs through the walls and running others through conduit), but it was a significant task. It added far more work than merely ripping the ceiling out and finishing the beams.
As for fire protection, I doubt that the plaster ceilings served as fire protection required by Code, especially if the previous removal work was done by legitimate contractors. I wouldn't ignore the possibility, and it is something to consider and look into, but I also would not be so alarmist.
Moreover, you can do retrofit work that serves the same purpose. I removed my ceiling to expose the beams, which exposed gaps that led to the space above my neighbor's ceilings. I fit vertical, custom-fit pieces of drywall inside the beams where they met the walls (continuing the walls up inside the beams) so as to create a firewall in each gap between the beams. (There is also concrete above the beams).