i cannot follow you gents in regard to reduced margins adding 50% wind to the vref on actual condintions and planned with zero wind for a given runway.
(a) we aren't adding the wind to the Vref. Generally, there will be a wind additive for steady wind (which is expected to be bled off as the aircraft descends through the lower boundary layer) and gusts (which is kept on throughout due to the random nature of gusts) with a total maximum additive (typically 20kt) to make sure that there is some sensible balance between handling and stopping ..
(b) the wind goes into the distance sums using 50% of the headwind (150% tailwind). It follows that the distance calculated for wind conditions is a little more conservative than for nil wind
(c) for a headwind, the distance required reduces. Main concern here is that one should make sure that the distance available is not squandered by excessive speed, height on the approach, floating, looking for that elusive smooth touchdown to impress the hosties, etc.
the runway has the same lengtht with or without wind
Quite true. However, for the discussions, we generally tend to be talking about the hypothetical limiting runway (ie nothing extra above the AFM requirement). In this case the required length for the headwind case reduces so that we might find ourselves attempting to land on a shorter runway which will require greater attention to getting it right ...