Thankyou
everyone for your input:
If I summarise everyone's input (considering that I've understood everyone's point of view correctly), following is the picture so far:
The logic of applying the factors is that we need some safety margins, since a normal flight by an average pilot is not flown the same way as a test flight by a test pilot. If the logic seems to be valid on ground then what makes it invalid in the air?
The judgement of safety could involve arguing why a factor was deemed safe before takeoff, but not necessary before landing.
So first we have to decide whether the safety margins are required during in-flight determination of the landing distance or not. How much is another story.
To start off with JAR-OPS 1.400:
JAR-OPS 1.400 Approach and landing conditions
"Before commencing an approach to land, the commander must satisfy himself that, according to the information available to him, the weather at the aerodrome and the condition of the runway intended to be used should not prevent a safe approach, landing or missed approach, having regard to the performance information contained in the Operations Manual."
If the regulators had intended a rigid application of landing distance factors in 1.400, they could have stated that intent explicitly. Since they didn't, the text the drafting committee adopted, leaves some discretion to national authorities to either lay down specific criteria for operations conducted under their jurisdiction, or to leave the matter to the discretion of the aircraft commander.
The issue revolves around safety.
Is it unsafe to takeoff without safety margins for landing at destination?
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?
If JAR-OPS 1.400 is about safety then the interpretation of safety by national authorities CAA and FAA is:
1) CAA: Operations on Contaminated Runways
http://www.caa.co.uk/docs/1428/20120...tedRunways.pdf
2) FAA: Landing Performance Assessments at Time of Arrival (SAFO 06012)
http://www.faa.gov/other_visit/aviat.../safo06012.pdf
Both authorities support in-flight application of safety margins.
In addition to above another reference that provides an indication about in-flight re-planning is:
JAR-OPS 1.475:
General
(a)An operator shall ensure that the mass of the aeroplane:
(1) at the start of the take-off; or, in the event of in-flight replanning
(2) at the point from which the revised operational flight plan applies, is not greater than the mass at which the requirements of the appropriate subpart can be complied with for the flight to be undertaken, allowing for expected reductions in mass as the flight proceeds, and for such fuel jettisoning as is provided for in the particular requirement.
An operator shall ensure that the approved performance data contained in the aeroplane flight manual is used to determine compliance with the requirements of the appropriate subpart, supplemented as necessary with other data acceptable to the Authority as prescribed in the relevant subpart. When applying the factors prescribed in the appro- priate subpart, account may be taken of any operational factors already incorporated in the aeroplane flight manual performance data to avoid double application of factors.
When showing compliance with the requirements of the appropriate subpart, due account shall be taken of aeroplane configuration, environmental conditions and the operation of systems which have an adverse effect on performance.
For performance purposes, a damp runway, other than a grass runway, may be considered to be dry.
An operator shall take account of charting accuracy when assessing compliance with the take-off requirements of the applicable subpart.
Landing — dry runways
(a) An operator shall ensure that the landing mass of the aeroplane determined in accordance with OPS 1.475(a) for the estimated time of landing at the destination aerodrome and at any alternate aerodrome allows a full stop landing from 50 ft above the threshold:
(1) for turbo-jet powered aeroplanes, within 60% of the landing distance available; or
(2) for turbo-propeller powered aeroplanes, within 70% of the landing distance available;
However several operators and many pilots do not interpret “
or, in the event of in-flight replanning” as being applicable to planning an approach, as in requiring a reassessment of the conditions for landing. Their point of view is that ‘
it only applies to diversion’.
i.e. inflight replanning is only applicable if you replan in such a way that you change your original plan with a new destination or new alternate. You therefore have to perform a new dispatch calculation if performing a replanning. Whilst airborne you need to perform an operational calculation before landing to satisfy company regulations (approved by the authority).
But over all I think everyone agrees that in an event of in-flight assessment/determination/planning or whatever the terminology is, some sort of safety margins need to applied above the actual unfactored landing distance.
If rules can specify precise factors for preflight planning then why cant they do that for inflight planning? IMO may be because inflight planning is much tighter than the preflight one (e.g. you cant offload payload, financial cost of diversions etc), so they've left it up to the national authorities who have left it up to the operators. And those operators who havent specified anything will take take advantage of the wordings in JAR-OPS1.400
"commander must satisfy himself" 
So moral of the story: "
make sure that you have a good story for the Judge"