Leaving it open is no big deal either. Or would that be the downfall of SO?
People come to StackOverflow because it has a good signal-to-noise ratio. Maintaining this ratio, and the reputation of the site as a resource rather than a discussion board is important.
I personally jump to StackOverflow results on Google pages because they almost always solve my problem exactly. If the results start being polluted by a bunch of less relevant discussions--which, being popular, are likely to rank well--I would probably stop using SO results as much.
On top of this, just leaving a question like this open hurts the question itself as well. There is no reason to want 100 different answers all pointing out the missing space in different words! That way lies madness and Quora--where many answers to popular "survey" questions only get read because of the follow feature and a Facebook-esque machine-learning-based feed.
"Welcome, Redditors! We're looking for answers that provide serious, practical solutions to the problem stated. If you'd prefer to post a joke or launch a tangential discussion, please do so on the corresponding Reddit thread."
Why is that not a suitable question?
I'm just perplexed by the trend over all, especially with Wikipedia, for people to act like the internet is a library and shelf space is running out. Why would a site owner limit their user's contributions in any way except to conform to the law, and prevent trolling, spam, etc? Who knows how much value the next answer might have provided readers if they hadn't closed it down?
This is not to say SO or Wikipedia has the balance right; but it's a sensible reason to "limit their users' contributions."