The Duck got it right!

There are several problems with this system

Two have been mentioned:

Too much tension, you'll rip the plant out.

Too little tension and it won't hold the plant up as it gets heavier.

These seem rather obvious once you think a little, but the thing that makes it totally useless is, as Lomcevak Duck said, Plants grow from the TOP

But I'll bet this guy sold a BUNCH before anyone figured it out