Any relational database can also do "schemaless" models, by using the EAV (anti-)pattern. Mainly this conveys a lack of understanding of your data and a lack of planning and design in your datamodel, but hey, it happens. The fun thing is that you still get all those nice database features like parallel processing, concurrency, SQL, ACID transactions if you want them, security and maintenance tooling, etc.
And if you use a modern database like SQL 2014 or Oracle's latest, you will get column-based compression (okay, it still sucks in SQL Server 2014, but it's a start), so the whole issue with extending sparse schema's is moot. If you use the 6th normal form it's not an issue anyway since that implements column-based compression by modeling it.
What you say is of course correct. It's just that for people who have a nice toolbox with all kinds of data models, relational databases go a lot further than most people think.