Part of the confusion comes from what the rules are written for.
The segments are, initially, for certifying the aircraft. Hence they are spelled out in detail in the FARs. Naturally if an aircraft is certified against certain requirements that manufacturer will have generated the flight test data to prove their case. Thus they can publish this data for the user and if we can fly the same profile and have certainty of the aircraft's performance.
If you are flying an aircraft with marginal performance then you will most likely have to fly the same segments to be able to ensure terrain clearance. If, however, you are blessed with additional performance then you could safely, from a terrain clearance perspective, fly a different profile such as accelerating early or retracting flaps early.
Just to add to the confusion we now take into account crew workload, automation, and noise abatement when designing procedures. So to the casual reader of a company ops manual it may not be clear why events happen at a certain point. Myths and legends then appear that 'you can't' do certain things.
For example, in Australia the requirement is to ensure 35' terrain clearance on departure or 50' in a turning departure. There is no regulatory requirement to fly a segmented climb but most operators have a segmented climb procedure. But this does not stop you flying a profile of your choosing if you maintain the required terrain clearance.