I'm not using a (small) tripod on a classical monopod; but it might be useful to free your hands for doing something else, during a short while. This doesn't mean that you should leave the whole setup unattended. Personally, as a safeguard, I keep always a strap around my neck while handling a camera with a monopod.
I have an old Manfrotto 682B which has a detachable foot, which in turn holds three thin metal rods that can be extended outwards as a foot to support the monopod as a standalone platform
Not the most stable construction around, a bit wobbly (to put it mildly), I wouldn't trust it to stand unattended with a body and big lens (e.g. 2.8/70-200)
But as mentioned it's handy in case you want to have your hands free for a very short moment
I e.g. at times shoot catwalk with one body and a 70-200 on this monopod, and another body with a short zoom hanging from my shoulder. In case I want to switch to the body with the short lens, I can leave the other one standing without having to try and keep it upright.
But I also have use an extra strap around my neck for the body on the monopod, despite the foot.