Lewis is also good in the wet, ...
Correction: Lewis is very fast in the wet. But, he makes mistakes in the wet and more often than not, ends up behind Button in wet races (despite being faster).
Also, in a wet/damp race, good strategy is a requirement, which we all know that Hamilton/McLaren are incapable of.
Button tends to make his own decisions in wet races, so McLaren's incompetence (with regards to Button), is negated.
...and Button has a lot of cars to overtake.
On a wet track, a fast driver can make mince meat of cars ahead of him. Wasnt Button in 21st place or similar in Canada...which he won?
On a dry track, I would agree that a good qualifying position is important. On a wet track (especially at Spa, where overtaking is possible), however, qualifying position counts for very little.