As a young boy, I totally wanted a sidekick like R2-D2. Yeah, he was cute—but he was also useful, and brave, and stubborn and loyal. What's not to like? Even as a youngling, I was struck by the whole idea of astromech droids like R2 being able to 'plug in' to a fighter like an X-Wing. A kind of co-pilot and in-flight repairman in one. That was an ingenious little addition to the movies that really was kind of understated. Like a lot of the technology in the movies, it was just presented in the 'background' and never expounded upon by the characters (which made it seem all the more 'routine' and 'realistic' within the universe).
In thinking about this, I suddenly realized that this little technological trick has never really been dealt with in the rules (at least not to my knowledge). And in all the time I've played the games, I've never really made much use of astromechs or their special abilities. What a travesty! These little guys are useful as heck—well, at least the cool ones like R2. Therefore, I'll do a bit of brainstorming on how astromechs might be handled in the game.
How I figure it working game-mechanic-wise is almost akin to the Jedi force power 'control pain'—in that the in-flight repairs that an astromech is able to accomplish are only jury-rigged and temporary solutions that remove the penalties for damage without repairing the actual damage itself. Thus, your X-Wing gets hit, takes 1 level of damage. Lets say this results in a 1D penalty to its maneuverability. Our plucky little R2 unit re-routes power and does some quick damage control the very next round—returning that 1D of maneuverability. However, the ship itself is still considered to have 1 level of damage on it.
Of course, there would have to be some limit to the droid's ability to repair systems. Perhaps something like they can only negate 1D worth of damage to a system—and once the damage level on that system reaches critical level, they can no longer repair it. Hrm. I'm still finalizing my ship damage system, so I'll have to think on this for a bit.
All of the above would seem to me to apply ONLY to those fighters especially equipped to incorporate astromechs (in those cool little sockets). Thus the fighter itself is built around having a droid around to do repairs on it. On larger ships (say a Naboo royal cruiser), Astromechs would also be useful, but their repairs would take longer—mainly due to the size of the vessel and the increased complexity of larger systems.
Oh, and no. In my Star Wars galaxy, Astromechs can't fly. At least, not in normal gravity conditions (zero-gee thrusters would make perfect sense considering the purpose of the droid).