We recommend simplicity over complex grammar. But it's not quite that easy, is it? It all depends on what you want to say, and on what effect you want to achieve - and that means choice. If you can get away with simple tools, fine. Use them. There's no point in using a pneumatic drill when a hammer will do. But sometimes you do need more powerful tools, and if you haven't got them, you're stuck.
At times you really do need a complex sentence - "complex sentence" in the technical sense, where it means a sentence containing a subordinate clause.
For example, the sentence you're reading now is a fine sentence whose complexity is justified by the complexity of the thought it expresses. (As you can see, each of the three relative clauses does an important job).
Sometimes a verb or a noun really does need a lot of modifiers. If you want to distinguish a quick brown fox from a slow black one, so be it - you need "quick" and "brown". That adds complexity, but it's justifiable because it helps to create a more vivid picture.
Sometimes a more complicated grammatical pattern is a real help to the reader. Take the very simple sentence: "Last night I had pizza with friends." What's the important or most relevant piece of information here? Is it the pizza, or the friends, or last night? Let the grammar become more complicated, and you get the answer: "Last night, it was with friends that I had pizza." (That's a cleft sentence, in case you don't know. We'll talk about them another time.) Sometimes complicated grammar actually makes the reading easier, so simplicity and ease of reading aren't the same thing. Compare these two versions of the same sentence: To see you looking so happy is wonderful.
It's wonderful to see you looking so happy.
Most people prefer the second, but this has more complicated grammar. (The grammar is called "extraposition".) Relative clauses, cleft sentences, extraposition and so on earn their place in any writer's toolkit. They help us to add subtlety and detail to our writing. It's important for young writers to be aware of these tools. The tools are already part of ordinary speech, so why not use them in writing too? This is where teaching comes in, as a way of encouraging the use of all existing tools.
But that's not the same as praising complex grammar. By all means give credit for exploring complicated new constructions; heap praise on those with large working toolkits. But it's choice that matters. (Incidentally, that was a cleft sentence.) Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk