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)?