(Hoping to not stir the pot. . . .)
While not an expert, by far, here are a few details about split wings from what I've encountered:
1. Split wings can result from injury, diet, and/or genetics.
2. It is most commonly believed to be an autosomal recessive (which is why it can crop up in closed flocks even after many years - if both parent carry "it", it can show up in youngsters down the road.)
3. Outbreeding with birds who are not carrying the ressive gene and proper selection based upon lineage can eliminate/greatly reduce its occurance. (One report took 8 generations with culling to effectively eliminate the occurance - a long time, but it can be done).
4. Details records are essential if its present in a closly related flock.
5. It can be related to the slow feathering K gene (common in cochins)
6. If caused by a dietry/management issue, birds can and do often molt out of it.
7. The issue can be in the primary feathers or the secondary feathers - there should be 10 of each on each side I think.
8. Judging split wings on chicks, young birds, recently molted, or molting birds is difficult. Best to wait until fully feathered with mature wing feathers.
9. split wings can be on one wing or both.
10. It can result from problems with wing structure (tendons and bones), feather follicles, or feather development.
I guess the best advice is to avoid it if at all possible if you don't have it, if you do keep detailed breeding records for many generations, breed from unrelated birds assumed to be clear of it, and consider outcrosses to help reduce it. Remember that even a pair of champion birds can produce split wing young if they are carriers of the gene!
Is it a defect? - Yes, (except for a few rare breeds) but it isnt the end of the world. Like all weaknesses it should be avoided, but it can be managed. As I'm not fan of heavy culling in place of good breeding, I would hesitate to maintain a closed flock with it. I would not be against outcrossing to control/reduce its occurance.
Dave