There are in fact growers who will cut the bottom off a plastic pot and just stick the whole mess on the surface of the new pot, letting the roots grow down in search of new soil.
This might be helpful to growers who have plenty of headroom but not a lot of floor space and want to maximize plant count in a small space; or, maximize volume to footprint ratios. I think the BOG started that fad.
It's also a good way to deal with a plant that's 'stuck' in a terra cotta pot- just break the bottom of the pot off with a hammer and put it On a new pot.