It's a concern of personal taste. Anyone in the future can contribute to threads that were made years ago without an issue. Let's say that aliens visited our planet today and we were to create a thread/poll discussing whether or not we should greet them with open arms. The year afterwards is peaceful and so on, but on the anniversary of their arrival to Earth, they take down our global infrastructure and put themselves in command of our species. This is a situation where it might be inappropriate to have a poll left open indefinitely. Whereas a poll discussing whether or not the US military should leave Japan would still be pertinent after several years of the poll's creation. Opinions may change, but polls in actuality have little importance to the discussion of the overall subject at hand.
My opinion? It's a matter of taste.