what "should" and what "should not" be - or what "ought" to be. is up to every single player himself. - there is no lore that suggests that a battleship, a cruiser, a destroyer, a bomber, a light fighter cannot or can fly effectivly on their own.
you have a valid view about lone warships - but it is just as valid as someone saying "they are powerful enough ( in RP ) to hold their own - without any escorts". nither is right or wrong. - so we cannot blame lone warships. - for we do not know what sci-fi philosophy this person follows.
maybe its like star trek, where warships fly very well all on their own. - maybe its more like Farscape where they are surrounded by dozens of fighters.
the fact that they are sitting ducks is not related to what they "should" do - its related to what precautions a character would do to avoid such situations. - but thats again up to the character / player ( if we re serious about RP, its entirely about the char ).