This question deals with a basic aspect of human behaviour, in my opinion best described by Douglas Adams.
The question is pretty much answered by ChrisF and heavyd already;
- SO users come across a question they don't quite know what to do with
- It's not a SO question, but it's not stupid either
- It's not about networking or servers
- Then it probably belongs on SU.
The bigger the community (or corporation) gets, the easier it gets to pass along a problem rather than deal with it yourself, because you're not in direct contact with the recipients of the problem. And if you have to read a lot of different faq's before you choose whom to pass the problem to, it gets worse.
We like to deal with problems as quickly as possible, and unimportant problems even quicker. So, bottom line, it should somehow be made almost as easy to pass a question that's irrelevant to one stack exchange site correctly on to another as it is to pass it randomly.
Maybe the faq's for the site in question should show up each time a user tries to pass along a question?