The weight you lift is a tool to reach your goals. It is not a goal by itself.
The general idea behind this rule is that you should not go into a workout in order to bench press a certain weight; you should go into a workout to look better or to be healthier or to get stronger. The indivdual exercises that you do is a means to an end and not an end in and of itself. The key is to know what your real goals are for each workout, and to lift the weight that will achieve that goal. Too much or too little and you will fail to achieve the actual goal.
This is hard to disagree with. I am not a weight lifter who runs, but a runner who lifts weights. I am always structuring my weight lifting around my running. What season I am in, what races I am running, what kind of mileage I am hoofing through, and even what specific running workouts I am going to do on what days have a huge impact on what I try to accomplish in the weight room. I try to never lift at the expense of my running, but I still want to lift enough to get the greatest gains. Those gains are not just in regards to my running but also for my overall health and strength. My appearance in the mirror is also slightly important.
The real key is to just know what you want to accomplish and why. Once you have that figured out, you can decide on what you need to do in an individual workout and on individual exercises.