This has been discussed, but in a different form. Generalization exists because there are so many people on the planet. I will list many examples of generalization and reveal the flaws of an adopted modern day attitude.
Insurance:
You pay a higher premium on your car because you are under 25. Your next door neighbor pays half of what you pay because he is 42. You are a higher risk because in "general", you have less driving experience. But who bothered to verify this? Maybe your neighbor with same car only drives it once a week for a few km's. Maybe you have had a drivers licence for a shorter period but have driven every day to a work destination far from your house. So he may have only driven 50 000km's in his life and you may have driven 150 000km's... But generalization dictates that he is the lower risk. The situation was not considered in it's own merit.
Bank Loan:
You apply for a personal loan, and so does you uncle Bob. You have a young credit history with very little debt on your name. Uncle Bob has mountains of debt and all installments are up to date. He gets the loan with a very low interest rate. You are declined because you are a higher risk or you are approved but at a higher interest rate. Who says uncle Bob does not hit bankruptcy the following month. Generalization dictates that the older credit profile will be the better payer.
Job Interview:
You have years of experience in the field you are applying for but no papers to back up what you say you can do. The next candidate walks in with hos diploma in hand. Guess who gets the job. You could easily have been the best asset that company ever acquired but they will never know it. Generalization dictates the one the "qualification paper" knows more than you do.
Drinking and Driving laws:
Everyone has the same limit. If you are over the limit then you are guilty. Arrested and charged. The law does not take the time to assess your alcohol tolerance. You may easily be able to drink 6 beers and still have perfect judgement, the next guy may be falling around after 2... Generalization dictates that you are intoxicated if the breathalyzer says so. Your tolerance for alcohol was not considered in its own merit.
Cheating Partners:
I do not need to explain anything here... It goes without saying. Just use what I said above and think about all the singles (cheaters) you have turned down simply because it is your "general rule of thumb". How many could have been "sex gods" or even really great people to connect with but you never took the time to consider each "application" in it's own merit.