Corpus linguistics has called into question the notion of fixedness itself.
As stated earlier, idioms and proverbs were considered to be the primary fixed expression to be learned.
Now, we see that it is the semi-fixed expressions that show some degree of flexibility are the ones that are particularly important to recognize.
They are in effect, the secret ingredient for preparing coherent speech that is often lacking in students learning new languages.
In fact, many times the students know more grammar than the teacher, but they cannot synthesize translated ideas readily enough to become fluent end proficient writers without this awareness of how and why phrases connect using lexical phrases.
They are the “glue” that holds everything together, and without them, there is no possible way to form coherent text.
Learning a ret of fixed phrases for situations restricts usage to only those situations, and does not allow expansion of the lexicon or enhance listening skills.
Learning lexical phrases does both.
They give a lot of bang-for-the-buck, so to speak.