I mean they're fun to write, they can add real meat to an idea and can help rope in a wild cast - but they rarely do what you want.
For example, in Arrested Development the writers are always creating intricate reasons to keep Michael Bluth (Jason Bateman) from very sensibly leaving his family forever.
We had this exact same problem with one of our main characters - Hedge. He was a logical, boring and pragmatic character in a loopy, cartoon world, and it really made the whole show 'click'.
But every now and again we would need Hedge to be a little loopy. Otherwise he just stayed at home and never did anything interesting (which admittedly was almost always the smartest choice).
The question was: How do you force a character to do something out of character, and still have it in character?
As it turned out, one answer was to find some kind of leverage to exploit.
A 'blind spot,' if you will.
In Arrested Development, Michael has a Blind Spot where his son, George-Michael, is concerned. So let's say Michael is once again about to leave - Not a problem; the writers just have his son mention how important family is. BAM. Michael stays for his son's sake.
Hedge already had a weakness for anything 'boring'. Boring hobbies, boring collections, boring activities - whatever it is, he's a sucker for it.
All we had to do was play that passion as a Character Blind Spot, and suddenly we could get him to do the most ridiculous things - as long as it allowed him to 'find his lucky lederhose' or 'complete his cork tile collection' or even 'get back in time to tape the ads'.
You can see how useful it is. Simply find or create a Blind Spot, and then even the smartest character can become stupid, single-minded... and malleable.
Once we knew the rule, we started seeing it everywhere:
- Think about how reckless the very responsible Marty McFly becomes when someone calls him a chicken.
- And how quickly Jim Hacker compromises his inflexible integrity as soon as a loss of votes is hinted at.
- Or Uchiha Sasuke, who would do anything for revenge.
For further reading on the topic, this article might be helpful.