Having bopped in those arenas occasionally during off peak times, I can say that most of those guys are NOT concerned about rank.
They tend to be the "Yeah! Our Team Rocks!" crowd. They honestly get a rush out of winning an undefended map....I don't understand it, but they do.
(In the interest of full disclosure: It only comes up every so often, but I have to admit that I will make a run at a good rank every now and then. I know it is utterly meaningless, but the OCD in me makes me feel obligated to see what I can do if I have a good run where I usually don't -- like Feb tour, when I miraculously had a good KPD in GVs. So yeah, i did run a couple bases. I am utterly ashamed, but like any addict I couldnt help myself.....)