We've no objection to other topics, but we've had enough trouble in the past with certain topics that get out of hand. Discussing other things is always a good way to practise, but some topics have a tendency to lead to trouble.
"learning English is much easier than mastering it"- I'll let you know when I've mastered it how difficult it is.
I agree with your point about exams. Unfortunately, we are so exam driven that a lot of teaching has become simply cramming for exams, but I'm afraid we're stuck with that. ;-(