I find that any work you put into, or have invited to an 'everything group's' gallery gets buried under a hundred more submissions within a few minutes of your art appearing in the gallery, so your work doesn't actually get noticed . To be honest, I get the feeling that those types of groups are managed very well either.
Think of it as a way of categorizing deviations to make them more available to the public. With DA's current system, one can only find the most popular or newest deviations, which means everything else gets lost amongst the test of time. With style-specific art groups, it's a little easier for your target audience to search through and find your work.