I find relational algebra most useful for anything involving querying, validating, aggregating, and transforming data. SQL99 is by far one of the more useful implementations of it that I know about. And yes it does have limitations.

That's why most mature RDBMS servers ship with at least one procedural language. Though it'd be nice if there was an option to use OCaml or Haskell in PostgreSQL.

I'm not suggesting to throw out all your code and build your entire application in SQL. I'm just saying that if you control the database, use it, exploit it and don't abstract it out unless you absolutely have to (because you need to ship your application on-premises to clients who may run MySQL servers and others who run Oracle.