Fabric surfaces were easier to repair after combat damage and damage to the fabric skin was less likely to risk structural failure to the rest of the airframe as in the case of aircraft such as the Spitfire where a semi-monocoque frame shared loads with the metal stressed skin. The fabric covered Hurricane for instance, was able to absorb a lot more hits from an enemy than the all metal Spitfire, especially in the case of hits by cannon shells.
As for control surfaces being fabric covered, I believe they were simple to manufacture (having skilled people and knowledge in these fields from the earlier days) and the ease of manufacture using cheap materials and construction methods outweighed the problems with purchasing and developing new metal working tools and rigs in wartime. Control surfaces made of wood and fabric were also very light and easy to balance, but as was said, did have disadvantages at high speeds.
It is interesting to note that a lot of (in particular German) aircraft were using fabric covered controls surfaces late in the war to save time in the manufacturing processes when the need for warplane numbers outweighed the need for very high quality construction.
Kermie
