Biplanes? Why not...later.
The Swordfish, the Gladiator, the Fiat and the Henschel.
To make them a bit more useful, maybe they could be launched from Vehicle bases, as spot planes.
Anyway, I don't think they would be completely useless, just not much used except in scenarios.
And regarding them being hopeless flying targets, I remember that being said about the Hurricane before
Whith more early war planes being brought in, it is just a qestion of time when we get those. And in a group of planes from the same time, they would not be so hopeless at all.
Well, time to kill with my Hurricane, see ya all.