If there is a pattern, it's that the person who put [citation needed] didn't necessarily agree with the preceding statement, but either (a) didn't want it to turn into a conflict or (b) didn't feel like doing the research and (perhaps rightly) decided that the person making the claim should do it.
I think that in the aggregate they turn the tone of WP into something that's very passive-aggressive. But individually they are harmless, just pointing out the obvious ("here is a statement that is unverified").
Where I see lots of [citation needed]s is in articles that tend to be biographical or concerning an artistic work or work of entertainment. Average Pop Star's #1 fan will copy a bunch of stuff from APS-fan-forums.org and someone else will come along and think, "what is all of this (crummy) original research doing here?"
If they deleted the material, other forum members will keep reverting. But if they add [cn], most people know that if they're going to remove that tag then they'd better have a citation handy.
Obviously in very popular or contentious articles, they don't stay there as long, because more people are willing to go out and find citations that match their point of view. The only way to trump someone you disagree with in WP-land is by finding more evidence. Which is exactly how it should be. So despite their passive-aggressive side, I tend to see the [cn] as a sign that the system is working somewhat, albeit slowly.
But seriously I'd nuke half of the articles on WP if I had the authority. WP can take page views away from a site that actually *is* accurate. And there's still copy-paste jobs going on. A WP article, by virtue of being able to draw from multiple sources and have multiple editors, should be more accurate than the sources it draws from. When it's not, it does people a disservice since it's going to show up first in Google whether or not it's any good.