Drupal seems keep a large amount of PHP serialisation of configuration in its database (a JSONish thing), probably because it's the standard data store for the site. Noticed this in fixing a dump one time. Got to use vim's function syntax for regex to ensure the character length count of the string matched the value of the new string.

I think virtually everything generically configurable in the UI that isn't part of a small core with dedicated fields (or in the sitewide PHP config include) is handled that way.

Most databases have some kind of special support for XML and/or JSON these days. And sometimes you just need to associate structured hierarchical data with each record without knowing the precise schema in advance. If it's non-hierarchical, you can always use a separate key/value table; but tree queries are not exactly convenient (or fast) in SQL.

Postgres is really happy with xml or json, as you please, and saves you from the complexity of having another data store.