There are various theories about more bass transfer into the pickup but in my experince it just seems to make p/ups sound brighter.EVH was one of the first and now several companies, Ibanez, Musicman etc employ it.Another consideration is it does away with the look/expense of a pickup ring but it also opens up the interior cavity and mean the spray job and woodworking has to be tidy too.
I think Eddie nailed his PAF onto a Strat body more through it being a quick and solid way of getting a humbucker where once there wasn't one :D