How much can you modify a program?

contingencyplan

I see a lot of posts by people who do not meet the fitness recommendations for programs like P90X and Insanity taking them on anyway insisting that you can modify, modify, modify to suit your fitness level. As a result they end up basically modifying the whole program from beginning to end. I appreciate and admire their dedication, but I have to ask...

At what point do the modifications defeat the purpose of following such a structured program? At what point are you better off just going for a jog and doing some basic circuit training with dumbbells? The structured nature of these programs exists for a reason and I can't help but think that people who attempt to tackle then modifying everything in the process are only making things unnecessarily hard on themselves and setting themselves up for failure.