If you go down at any reasonable speed, your knees are going to get tore up, with or without jeans. Jeans are not tough enough, unless they are kevlar-reinforced. You'll still be going to the hospital.
Come out to Malibu tonight and look at Mike's knees. He went down in jeans 2 months ago. He is the guy with the yellow 911.
I ride in jeans sometimes. I'm not saying don't do it. I'm just saying don't kid yourself. Any jeans other than Draggin' Jeans aren't going to do anything about protecting you.