I think this is a bad idea, and here are a few reasons why:
(1) Increase in Noise. I believe the Ubuntu Forums have become the de-facto place for that sort of discussion in the Ubuntu community. In fact, the Forums are so full of silliness that I don't find them useful anymore. This quote from the six guidelines for great subjective questions expresses it well:
Most forums and chat rooms have a scale problem. As in, they don’t. The more people that join the discussion, the more noise each of those connections bring. So the forums get progressively noisier and noisier, and suddenly one day … you stop learning.
Ubuntu Forums is no longer a place for learning. People are lucky to get an answer to a serious question there because their voices quickly get lost in a flood of chatter, redundant questions, games, and other noise. On its own, the StackExchange system can only slow the progress of noise; moderation is still necessary to eliminate the noise that inevitably creeps in.
(2) Decrease in Signal. Likely a result of #1, it seems lots of the people who know what they're talking about have stopped checking in at Ubuntu Forums. There are still knowledgeable people there, but I believe ensuring Ask Ubuntu is an on-topic, learning-focused atmosphere will make it the easiest to find and keep knowledgeable people around.
(3) Monkey See, Monkey Do. It will probably be the case for a fair while that the majority of Ask Ubuntu users aren't yet used to the StackExchange system and don't understand why it's set up the way it is. More for-fun-only posts means more dilution of the message about what kinds of questions should be asked (especially when moderators and those with high reputation participate). This will likely result in an unmanageable portion of Ask Ubuntu users thinking "…hang the rules. They're more like guidelines anyway."
So, am I saying Ask Ubuntu should be completely serious one hundred percent of the time? Absolutely not! Humor is vitally important to human interaction. What I am saying is, let's encourage humor and whimsy as a part of a good question or answer, but discourage and remove posts when it's the substance or focus of a question or answer.