My Gitzo tripod (can't remember the model) has a central column. I rarely use it to extend the height, because, as others have mentioned, stability issues.
However, for macro work, reversing the column and extending the legs out, works a treat. In fact 90% of my macro work is done that way, as I'm usually photographing things low to the ground. My second tripod, when I get around to buying one, will be a more solid unit without the center column, for tele lenses and landscape work.
Horses for courses really.