When is a Tripod Actually a Monopod?
When you extend the center column in an attempt to make the tripod taller.
Think about it. A tripod works because it has three points of support, but the moment you extend the center column, at least a portion of your tripod is now only supported in one place. I see people doing this frequently; their criteria for buying a tripod is one that isn't too big and heavy to carry around, but when they go to use it, it isn't tall enough, so they pull the center column way up
You are much better off with a tripod that is sturdy, and that generally means it's also heavy. If you do compromise and get one that's lighter and therefore probably shorter, resist the temptation to make up for it with the center column. You are better off stooping to its level, or buying one that is tall enough to begin with.