I think it is just something Boeing build in as a 'take it or leave it' option, We use 20,000' up and 20 minutes to go, which I think is a tad excessive, but if that is what the book says and that is what the cabin crew expect, then that is what must be done. The important thing is to follow the manual because you lay yourself open to disciplinary if someone gets hurt due to avoidance manoeuvre or weather.