If we're interested in their relative wet/dry performance the sample is the races they've won, not the total races!
So are you suggesting that we wait until the pair of them have won about 25 races between them? If we do this, we may never be able to use statistics to compare 2 drivers. We need to use whatever data we have at present and not wait for drivers to retire...by that point, it'll be too late and we won't even be talking about Button or Hamilton.
The facts are as follows:
100% of Button's wins for McLaren have been in the wet.
100% of Hamilton's wins for McLaren (since Button joined), have been in the dry.
Make what you will of that.