I believe that the special roles are meant to limit what ships are faced in campaign missions that select ships based on BPV rather than scripted vessels; it is set up such that you shouldn't face, say, 3 K-C9's; but you might go up against a K-C9, K-D7L, and K-F5L (all have "C" - Command as their special role).
It does go further than that, but I am not sure that they were fully implimented. In SFB, unless otherwise stated within a scenario, a carrier ("V" - Carrier special role in SFC:EAW and OP) is supposed to have escorts (E - Escort special role). Scouts, Maulers, Commando ships, Drone ships, etc. are supposed to be of a limited number (one each, iirc) per fleet.
The most handy would be "R" - Restricted, which allows for a ship to be available if scripted into a scenario, but otherwise is unavailable to you or the AI. This can be used to limit certain ships that you are having troubles with from being an opponent, getting dropped into a ship that you really shouldn't ever be in when you lose yours in a mission (F-CLH, anyone?) and allow for a special ship to be available for a special scenario, yet still be available in skirmishes and such.
As to the fests, I don't think that a sepcial role has much play there, but I didn't write those scripts, so I do not know for sure. Heck, most of what I have written here is based more on observation than hard facts.