The cheaper tripods are a lot ligher, so less stable. Especially outdoors - you don't want to see your camera + lens fall over and break.
Also, i think stability is key on shots where you have a very slow shutter speed so there's no movement (shake) in the final picture.
I may be wrong, but this is what makes sense to me..
Fair enough, but my "cheap takara" is solid as hell, with my k20d + 300 EX on it, i have tested it durability alot, and theres NO way itll fall over. Just my 5cents, the legs are as thick as, erm, my tv remote control.
I don't think I can afford 4 kilobucks for a "pro" pod now anyway, this will just have to work for the 4 times a year i use it