It all depends on the pilot's skill really. Depending on how they are able to work them, they can stop enemy planes from killing your allies, kill a bomber or two before they're ready to drop their loads, or even strafe a few people that are going to take a flag. I will admit however that people do think a bit too highly of planes, but even if they don't know how to use them properly, they can always just use them to drop into enemy bases.
Overall, I would say that while they only play a minor role, good pilots can turn a tide.