I'm not a big proponent of schemaless designs, but they have their place. One particular place where I think they are useful is for archiving of data
where even though the underlying table structure of the data you need to archive is changing, you want the archived record to have the same fields as it did back then.
This is a case where I think Hstore and the way PostgreSQL has it implemented works pretty nicely.
Side note: one of the new features of PostgreSQL 9.4 is improved GIN indexes (faster and smaller) which
is very often used with hstore data (and the new jsonb type). We're really looking forward to the GIN improvements more so than the jsonb feature. We're hoping to test out this improved index functionality with OpenStreetMap data soon and compare with our existing PostgreSQL 9.3. OpenStreetMap pbf and osm extract loaders (osm2pgsql, imposm) provide option for loading tagged data into PostgreSQL hstore fields, in addition to PostGIS geometry and other attribute fields. So 9.4 enhancements should be a nice gift for OSM data users. More on that later.