@Haroon;
I think you've summed it up quite correctly.
You've also added a couple of questions:
Is it unsafe to takeoff without safety margins for landing at destination?
It's not necessarily unsafe to do this as long as the conditions doesn't change enroute, but it's not legal. Definition of safe is not a regulatory issue. The regulatory agency has made the definition of safety up to national agencies/operators/commander to determine. For instance; one operator could get approval for one type of inflight assessment based on their training, type of equipment, experience level etc etc etc, whilst another operator would not.
Is it safe to land without safety margins in case the surface conditions at the time of landing are different from those planned at departure?
Not necessarily either. Not advicable perhaps, but not necessarily unsafe. Again see the answer above. Say your LDA is 7000 ft and your calculated unfactored landing distance is 7000 ft. If you fly your aircraft exactly how the unfactored landing distance was calculated. Vref+0 at 50 ft. Touchdown 305meters down the runway, application of maximum wheelbrakes 1 second after touchdown, deployment of reverse 2 seconds after touchdown up to full reverse down to 60 kts, before using wheelbrakes to come to a complete stop. Your nosewheel should be at the edge of the asphalt at the end of the runway. Certainly not unsafe. You can taxi fat and happy back to the gate, and all passengers are safe and sound without any injury to pax or damage to the aircraft. ARE MOST PILOTS OK WITH THAT ? Certainly not. How much margins you add are up to you, but I would certainly consider; Worn brakes, MEL, long days with fatigue, unreliable weather reports, unreliable friction measurements, worn tyres, increased ref speeds, pilots experience level, light conditions, and on and on and on. How much margins ? If using the dispatch margins you've given yourself a lot of margins. I would even say too much margins since you would have problems even landing at all applying all those factors, but you're not wrong to do so. It's all up to you. You can even add more margins if you deem it necessary.
If rules can specify precise factors for preflight planning then why cant they do that for inflight planning?
What do you mean by inflight planning ? Planning ends when the aircraft is moving under it's own power for the intention of flight. You do your planning on the ground before commencing your flight. If you receive information enroute to your destination that your alternate airport is closed due weather, you need to perform a replanning. A replanning must meet regulatory requirements, hence a dispatch calculation for your new alternate must be successful.
Rules or rulemakers do not specify specific criterias for inflight assessments. They require you to have the methods/SOP you intend to use approved. That's why you sometimes see some operators land and some don't because their approved criterias are different.
