A monopod is useful in tight spaces, like shooting a fashion show with hundreds of other people crowding around you.
It will not be possible to "open up" the legs of the tripod in such a situation.
A tripod with one leg extended to function as a monopod sounds good, but the diameter of the tripod is quite thick with all 3 legs closed and only 1 leg is extended to support it.
this may cause the tripod with 1 leg extended, to be very unstable and hardly usefully at all and if you have tried it, I find the photographer will be more worried about the tripod falling over than taking a good shot. His mind is not on the subject but on the unstable tripod.
I have tried it before, and it is very wobbly.
After all, a monpod is only $60-70, and you can use the tripod's head, so why not get a monopod instead of using a tripod as one?