I have heard a number of really memorable illustrations, that I cannot remember what it was they were supposed to illustrate.
(One about a cane toad in an electrical sub-station comes to mind)
An illustration must make that which it is trying to illustrate memorable.
If it won’t do that, and particularly if it is likely to make people remember it, and not what it’s supposed to be illustrating, then it shouldn’t be used.
The first necessity of illustration is that it should be true, that is, that it should have real relations to the subject which it illustrates. An illustration is properly used in preaching either to give clearness or to give splendour to the utterance of truth.
But both sorts of illustration, as you see, have this characteristic; they exist for the truth. The are not counted of value for themselves. That is the test of illustration which you ought to use unsparingly. Does it call attention to or call attention away from my truth? If the latter, cut it off without hesitation. The prettier it is the worse it is.
Phillips Brooks, The Joy Of Preaching, Kregel Classics, 1989, pg. 132.