I just took a look at a comm I moderate. Even on a computer (though admittedly small screen), the checkboxes already fill up most of the screen. More importantly, though, is the fact that not only would there be a lot more checkboxes horizontally (scrolling sideways, yuck!) but also there's a lot of members. When membership starts to paginate and you have to keep scrolling down, that's a lot of people to scroll/click through horizontally and vertically to find the correct person and give them the right privs. I'd rather be able to say "I want to deal with these specific pool of people and only look at them now to give them privs".