Seriously, that's your opinion above? I have worked exclusively restoring WWII warbirds for 17 years. More fragile is more fragile. It doesn't mean it isn't airworthy; it means it gets damaged more easily than another more-rugged airplane when treated the same way.
If they don't remove a dent or two or three, it doesn't really matter much. If they don't remove a lot of dents, it won't fly as well as it did when the skin was smoother. It certainly won't be as fast, after awhile, as one that is less dented, but it also doesn't mean it isn't fit to fly. I'm taking piston fighters here, not Eagles or Typhoons. I also didn't say Spitfires got slower as a group over time, but they DID require more minor repair/TLC than some aircraft that were built a bit heavier.
A P-51D weighs about 7,635 lbs. empty. A Spitfire IX weighs about 5,000 lbs. empty. They both sport a 2-stage Merlin. In my opinion, the extra 2,635 lbs. makes the P-51D a bit more rugged than the Spitfire. Your opinion may vary.
I'm guessing Spitfires lasted as well as any other WWII production fighter, albeit with a bit more minor maintenance required than some other aircraft did. Nothing out of the ordinary, and the Spits gave more than just good service; they gave excellent service. Nothing I said above detracts from their accomplishments. It's just a different animal than a more rugged one is; and is not better or worse because of it. British aircraft have always been good performers ... well ... maybe not the Roc / Skua family, but most have been very good airplanes. I'm sure someone can come up with one other than a Roc/Skua that wasn't quite up to snuff. I daresay that all countries that produced airplanes have had one or more that wasn't top notch. Can you say, "PZL Zubr?"
The Spitfire isn't among the less-than-top-notch and I never implied it was.
Saw something above about the A6M being weak. It isn't. Yes, it has .032" skin in places where some others have .040" skin, but it isn't weak at all. Most U.S. fighters were designed for 8 g's with the 12 g ultimate load. That's a 50% safety factor. The A6M was designed for 6 g's with a 12 g ultimate load. That's a 100% safety factor and both have the same load limit, but the A6M might and likely will sustain damage at 8 g's without failure where a U.S. fighter would not sustain damage in the same 8 g load. The difference is relatively meaningless since we all know a WWII piston fighter won't sustain even 6 g's for all that long. If there was an A6M that lost a tail in trials, you can be sure it was damaged or over-g'd. The A6M is a very good airplane that was purposely-designed to have the general performance of a 1,500 hp Allied fighter with an engine that never developed more than 1,160 hp. It had to give something up to get that performance, and what it gave up was structural robustness.
If everything was undamaged, the A6M was quite strong. But, if damaged structurally, it didn't have the same reserve strength that a more heavily-built airplane enjoyed. If you were in an A6M and took a 50-cal bullet to a wing attach point, it was likely nylon letdown time. Perhaps so and perhaps not if you were in a P-40. At least you had a good chance of flying the P-40 home. Call the P-40 bad names if you want, but you can't say it was weak or that it couldn't absorb damage well.
Can't say the same about the ability of the A6M to absorb much damage.
None of the above addresses the Mosquito thread title so, if we MUST discuss this further (do we have to?), maybe in another thread or PM? Cheers.