There already is a major problem with people posting in threads that have long been dead (strategy, tips, tactics, etc. for civ3 that was written well over a year ago and made obsolete by patches). Some people don't pay attention to the date on a thread. People using the 'search' function find lots of outdated threads and if they also post in those polls, the threads would be brought back from retirement again, restarting the whole debate all over again.
If polls are going to be brought back to the top of the list evertime someone votes in it, then there should NOT be an option to run the poll forever. Threads that are over a year old (maybe even 6 months?) should be closed to prevent someone bringing the thread back from the dead.