jquery-mobile-960 are not just the grid. this is a grid inside jquery-mobile. If you just need a css grid. less framework is a viable choise
Given that I have to include jQuery for using Less Framework anyway, isn't it always better do just use jquery-mobile-960 instead?
well jquery mobile is a lot more than jquery.
in very short, jquery mobile is like jquery+jquery UI but on mobile. For more details
http://jquerymobile.com/