Talking about star wars, in RoTJ rebel command orders to keep enemy fighters away from capital ships. This means that they must have been a reasonable threat to them.
The Rebel fleet contains a significant number of relatively light warships, at least judging from the size of the ships. It also contains at least a couple of ships which at least appear to be of the same class as the transports used during the evacuation of Hoth, suggesting that not all of the Rebel ships present at Endor would be frontline combatants if the Rebellion had the resources to field something else. I also note that we don't actually see any TIE Fighters, TIE Bombers, or TIE Interceptors firing on anything bigger than the
Millenium Falcon in the entire battle sequence, despite the comment that there are "too many" Imperial fighters in the view of at least one Rebel fighter pilot, suggesting that at least some fighters should be available to make attacks against the large warships (nor, for that matter, do I recall seeing any TIE Bombers in the engagement, which from the name might be expected to be used against heavy ships, especially since they're clearly not designed for atmospheric use, despite there supposedly being something like a dozen of them per Star Destroyer and the TIE Fighter and TIE Interceptor armaments being exclusively fighter-grade laser cannons, as far as is demonstrated in the movies). The only dialogue indicative of attacks against large Rebel warships are that comment about drawing fire away from the cruisers and a later comment about fighters heading for the medical frigate; under the assumption that ship classes (destroyer, frigate, cruiser, etc) follow roughly modern naval patterns, a frigate is a rather light vessel and could therefore be expected to be relatively vulnerable to the light weapons carried by Imperial fighters.
Moreover, there are more reasons than just that the fighters pose a credible threat to the capital ships for why you might want the fire of the fighters to be drawn away from the cruisers. We know from the first Death Star run that fighters can at least disable some weapon emplacements and cause minor surface damage to (presumably armored) hull, and we know from the Endor battle sequence that fighters are capable of taking out what appear to be sensor domes (which are likely at best lightly armored; EU claims and a confusion of cause and effect have made the domes into Star Destroyer shield generators). Fighters have not demonstrated an ability to destroy capital ships in Star Wars (barring incredible luck), but they have demonstrated the ability to cause damage which might adversely affect a heavy ship's combat performance. We also know that at least the Rebel fighters are capable of concentrating their shields in a specific direction or setting them to a more even distribution of protection; if the capital ships are likewise able to concentrate their shields, there is an obvious reason to want the fighters to be dealt with - keeping them from strafing the cruisers allows the cruisers to concentrate their full shield strength in the direction of the enemy capital ships without needing to worry about suffering minor damage which poses no real threat to the ship but nevertheless adversely affects combat performance. Then there's the issue that even if fighter weapons are not nearly as potent as capital ship weapons, it's still energy that the shields have to deal with if the shields intercept the shots, and when you're already dealing with a collection of heavy warships which apparently outclasses your fleet ("we won't last long against those Star Destroyers") every little bit probably counts.