It depends. The captain may not excel at combat but might know how to swing a sword adn every bit can help. He might also wait outside the entrance to the dragons lair. It is very dependent on context. I have certainly had bards in the party who weren't much in combat or even much out of it but participated in combat and wrote epics about the combats. More often though such haracters have something to contribute.
But old school d&d actually doesn't make that many assumptions about party composition. You used to roll your attributes and stick with your results. So it was entirely possible to have some feeble characters in the party. Usually this sort of situation is where the player got clever and found an angle like cook.
My point isn't that players should play boat captains or zero level characters, but that I dont want the designers making too many assumptions about how
People play D&D. They did that with 4E, in my opinion a bit arrogantly, and the results were not fun for about half of the gaming population.
My point is that the character needs some reason to be in the party. After a certain point, a PC has to fill an important role of the party or the other PCs wont have a reason to keep them around outside of personal feelings. Companies laid off, retire, and fire employees without purposes.
Ineffective characters don't match my idea of a D&D character. I prefer the designers make sure that every class and race have a point of existing as an adventurer.