Who the hell said that the GTD Delacroix is an Orion!? For all we know, it could be a Hecate (or some other unmentioned class of Terran destroyer). In which case, the Ravana would have it for lunch, especially if it were a Hecate and was attacked from the rear/side where it hasn't got a single beam cannon to fire back with.
The Ravana is a purely offensive weapon. Having it try to guard something (or even itself) is futile, since it was never meant to do that. It is meant to jump in, roast a ship, then jump out. Preferably before its beam cannons are destroyed. It works WONDERS doing that job (well, assuming there are no omniscient fighter pilots who know exactly where it will jump in and take out its beams the moment it does).
If the Shivans needed a destroyer to defend something, I think they would use a Demon...
During The Great Hunt (FS2), the Shivans used the Ravana well. Jump in, roast a corvette, jump out. Although, on the complete opposite side of the spectrum, in the next mission (Slaying Ravana) the Ravana was retarded. It sat there, and sat there, and died.