Except that wind speed is given 10m above the runway, the speed 10m down can be significantly lower. In a 737 etc that makes little difference in a spamcan it is a huge difference.
You only have to look at the AIB reports to see the number of people who have made this mistake on short fields.
There is no way I would want to be whistling along the runway at barton in a cherokee at an IAS of 75kts regardless of the headwind component at 10m......
As has been pointed out several times in this thread, the additions work fine for large aircraft on large runways, on a small aircraft the type of increases you are talking about are massive increases over POH values.
Perhaps the reason why airline pilots often struggle with a return to light aviation after a period away.............