A good use for dead container plants?

So I had a Ceanothus in a pot that died. After the funeral and 21-gun salute had concluded, I was thinking of what to do with it. Rather than take it out and compost it, I instead stuck an elderberry cutting right in it, without doing anything to the pot. No rooting hormone or willow water either. And the cutting has begun forming leaves! Hopefully it'll stay strong.

I've got a couple more cuttings in regular pots that didn't take, and I dipped those in honey solution. It makes me wonder if the root system underground that the ceanothus left is helping the elderberry cutting stay hydrated. Plus the dead ceanothus is good mulch.

Has anyone else tried this?

I find pots like this when unloading the garbage at the transfer station and slip them back in the trailer with the garbage cans. The root structure in the potting "soil" seems to make it more capable of acting like soil.

Howdy!
I had never heard of any particular potted plant being better or worse than another for such a thing, but I'll happily second Hans Quistorff's observation that just the fact of there being some sort of root structure of something does seem to send a signal of "Here's a Good Place to grow."
I have a couple of mulberry sticks that I had thought were dead, but I stuck them in with some barely rooted bay cuttings and everyone seems much happier.
Maybe the tree cuttings just wanted friends?