I think you need to separate the aircraft which crash because they are over weight and the aircraft which are found to be overweight after an accident which had nothing to do with being overweight. In the case of the Oban accident the aircraft was overweight but it is very unlikely it contributed to the accident.
I think a lack of planning tends to kill people. Lack of planning increases the likelihood of being overweight and out of balance, but it also leads to flying in poor weather, not doing a good DI, etc etc. Added to this is that some pilots have not done a W & B since qualifying. I use Navbox for my w & b and have a print out of the graph in the aircraft for refrence.
As to how frequently people fly outside W & B, it is hard to say with any accuracy. You can rarely tell by just looking as the aircraft parks up.
Rod1