Even the illustrations that Christ used were not meant to be easily understood without explanation.
Often times an illustration demonstrates to me how well a person understands a concept because the illustration they use is either apt to what they're trying to describe or completely misses the mark. It's not that the person utilized an illustration it's just that the illustration chosen was very poor because the person didn't really understand what he's trying to illustrate.
For that matter, then, a person could well utilize a Scriptural illustration (i.e. the Prodigal Son to illustrate the love of God) but, if his understanding is poor, then he'll misapply the illustration. It's not the story of a boy repudiating his father's house, spending all his money, returning in disgrace, and being welcomed back in open arms that really communicates the nature of God's care for the lost but it's how the teacher unpacks the story to draw the appropriate inferences from that story.