This practice been done by many production sites over the years http://backchannel.org/blog/friendfeed-schemaless-mysql Might not be considered bad practice now.
https://github.com/perfectsense/dari
Here is the SQL schema: https://github.com/perfectsense/dari/blob/master/db/src/main...
We've used this model for almost five years now with great success. It's simplified rolling out "schema changes" since no tables need to be changed. It's also been optimized to a point where it's extremely fast.
But every time a developer sees an interesting twist on a piece of technology and goes for it, peers call it a bad practice.
I've been through many cycles like this, and inevitably some time passes, and one day you wake up to see yesterday's bad practices have turned into exciting advancements.
Moral of the story is, ignore the wisdom of the day and go for it, tiger. Stuff that JSON in an SQL table.