In my years of arguing with people online (#xkcd386), I notice that those who resort to metaphors and analogies often have the least convincing arguments. Metaphors are great when you're explaining a difficult concept (of which you've attained a satisfactory level of understanding) to a willing listener, but it's really easy to enter the slippery slope you mentioned if the correctness of something is in dispute.

These days when I see a argument based on metaphor, I just ... disengage.