I suspect many posters remove their questions because they become frustrated w/ email announcements of new answers being reported. If the QP knew they could easily turn off the alerts, I believe we would have fewer questions deleted.
I agree that the answers w/o the questions creates a disjointed chaos. The flip side is that there might be a reference point in the thread for which someone may later search. i.e. Someone wants to find the fable about the dog that JR posted on some thread. If entire threads were deleted, this would cause one to search extensively to no avail.
My alternative suggestion for Trulia............. Create a dialog pop-up box that appears when a QP asks to delete their question. The pop-up would ask the QP if they wished to delete their question, or turn off the email alerts. Surely, some will still delete. Others may be quite content to leave the thread intact, but rid themselves of repeated alerts.
My 2 cents, plus.