The gre quantitative is not true. Philosophy is the best overall but just not that high on quant.
http://webb.nmu.edu/Departments/Philosophy/SiteSections/Resources/GRE_Scores_by_Intended_Major.pdf
I don't want to come accross as though I think studying Philisophy is useless by any means. It's actually among several topic i wish i could learned more about as an undergraduate and iff that's what you love, fracking study it. However, comparing test scores by major unearths interpretive issues with causation vs. correlation.
You can't take a fact like "philosophy majors score the best on the gre exams" and conclude that studying philosophy causes people to test better on the gre's. There is simply a correlation between the two variables. To assume that this is causal simply ignore the plethora of other variables, most important of which is the simple fact that certain types of people pick certain majors for specific reasons. No one would choose to major in math if they aren't good at math in the same way that no one would choose to major in philosophy if they aren't good at writing and reason.
What i'm getting at is, like any standardized test, people are predisposed to performing a certain way and their choice of major simply reflects that predisposition and does not necesarily cause it. If gre's were given to college freshman and arranged by future major, the scores would be lower but the rankings by major would be very similar.
Both math and philosophy majors will learn or practice skills during their undergrad that will help them on the test but that doesn't erase the fact that their choice of major is indicative of a clear predisposition to doing well on certain sections vs others.
tl;dr The type of person who chooses to major in philosophy probably would've done very well on the writing gre's anyway no matter what he did throughout undergrad.
edit- I just realized that the table i posted is arranged by intended graduate major. Perhaps the data would look a little different if arranged by undergrad major because there are a handful of people who go to grad programs for something different but probably not by much. For all intensive purposes, that doesn't affect everything else i said.