Why are some (higher end?) tripods sold without a head?
I've been trying to find a nice (size, weight, weight-capacity) for my new 7D. The last tripod I bought in 1994 was an aluminum unit. It was massive, with wide-diameter tubes and a flat slab with a screw mount for the camera. It had two arms - one for up/down, and the other for left/right. Now, when I can find reviews or good info, most tripods are the legs and the upright, with the need to buy a head of some sort. What advances do this "2-piece" setup of a tripod + head have over my older type of an all-contained piece?