I've only once tried to use stored procedures in mysql and it was almost impossible to debug back then. Very painful. Average devs already have issues being smart with their databases and stored procedures would add to that.

Stored procedures also add another risk. You have to keep them in sync with code, making releases more error prone. So you have to add extra layers of complexity to manage versioning.

I can see the advantage of extreme performance/efficiency gains, but it should be really big to be justified.