What subjects benefit from textbook editions? I've read several questions relating to the (typically US) practice of requiring students to buy up-to-date, expensive, editions of textbooks. I can see why the publishers are in favour of this, but I don't understand why everyone else plays along. As far as I've seen, the UK seems to get along fine without this.

In my field (mathematics) it seems pretty obvious that new editions are generally not that important - maths just doesn't change that fast (the material taught at undergraduate level has mostly been around for the odd hundred years). So my question is:

Are there subjects for which it is important to have the most up-to-date edition of a texbook, enough to justify the cost to students (/libraries)?
Take undergrad economics. While standard micro- and macroeconomic theory likely doesn't change quickly enough to warrant a new textbook edition every few years, students might be... irritated... if recent economic events (the US housing crisis, the Great Stagnation, right now the ruble meltdown) were not reflected and discussed. 

Suppose the last example of a major crisis in your econ textbook (printed in 2004) were the dotcom bubble bursting in 2001 - today's college students were barely walking back then. This would be ancient history for them.

Yes, of course a motivated instructor could work with an older textbook and provide the updates based on his own notes. This is a lot of effort, though, and apparently few instructors go to this trouble.