I recently arrived at the rep of 15Ki (where i stands for imaginary, per Dan Bron), and so I am allowed to protect questions. (Chortle!)
My rule of thumb has been to protect an old question that popped up on the front page if a new, unregistered user answered it with an answer that I considered VLQ or Not an Answer and if it already had good answers. I haven't gone looking for them.
When I started to ask this question, I saw a similar earlier question, When is a member expected to protect a question? with this answer:
1.Questions that are receiving a lot of poor answers from low reputation (less than 10 reps) users should be protected.
Point of clarification: Why should we wait for a lot of poor answers? Why isn't one poor answer enough to protect? Poor answers might attract more poor answers. Nip them in the bud.