SP:s was never mentioned in the post you are replying to, but ofcourse if you use dynamic SQL in them (execute immediate in Oracle) it is just as bad as doing string concatenation in other languages. Just dont do that.
SELECT sp_enroll("bob\"); DROP TABLE students; --");
SELECT sp_enroll("bob"); DROP TABLE students; --");
At least if you grant the user only SELECT and EXECUTE privileges and define the procedures using the SECURITY DEFINER property, you could still prevent this type of damage. (This relies on the procedures to be as strict as possible or the whole scheme essentially fails.)