I guess I'm not sure exactly how knowing the precise speed of the wind where you are presently kayaking really matters.

First of all, the direction and fetch and water depth and coastline features and everything else all add up to what "the conditions" are.

Secondly, when you're actually out paddling already, whether you're handling the conditions or not has zero dependence on what you measure the speed to be.

Thirdly, how will your precise measurements from other trips be of use--according to your logic--in predicting whether or not you will paddle a given stretch of water, for as you've noted, any weather forecasts will be for the area in general, and not for the speed right at your kayak. I suppose if you're standing on the beach with an onshore blow, you could measure that and compare to prior experience. But then again, your senses should tell you all you need to know about those current, local conditions.