Fixed and Semi-fixed Expressions or Cliches?

Students are rightly encouraged ro learn fixed and semi-fixed expressions, like the two sides of the same coin found in the Site Intro. Serious writers, on the other hand, avoid cliches like the plague. Is there a double-standard at work here? Should there be? Are learners (including children in their own tongue) condemned to absorb this massive database of prefab lexical chunkage only to later shun it? Is there a line to be drawn somewhere between common, useful expression and cliche? Where then lies this line and how are students expected to see it? I have no ready answer, but it feels worth worrying about.  :)  Comments?