simplicity, for one - write a quick query to retrieve all recipes that contain all of any number of ingredients.

  SELECT * FROM recipes r
   INNER JOIN ingredients i ON (r.id = i.recipe_id)
   WHERE i.ingredient IN ('mushrooms', 'sour cream')

load up some data and run explain analyze on both schemas/queries.

I could think of denormalization; e.g. you have an entitiy that can have 0..n tags and you want to speed up the retrieval, I could think of arrays being a faster way.