It's a lot like elections. Only in the case of elections, polls have high participation and the results are flung around as though they were gospel fact. Then the election happens, and the pollsters wonder why their "infallible" result collapsed into so much dust. The reason? 20% of the electorate actually went out and voted for real.
When polls are conducted on a forum, your best hope is about the same. The results are a lot like an election. You can have 5,000 members ( yeah, if only ) and post a poll only to have 100 of them vote. Obviously this is far worse than the 20% for a real election, and it often results in the appearance of an elite few who determine the fate of many.
None of this changes the result though - in all honesty I don't want to invest the time in creating an account system only to later find out it becomes the #2 most removed body of code after release.