I crunched some numbers using the
mortgage calculator. From what I can see on a $200K mortgage at 6% interest you would save $90 per month to extend it from 25 years to 30 years.
Now for somebody like Dominic who is looking to flip a place; it's fine. However if the difference between a 25 and 30 year mortgage is the breaking point between being able to afford a place or not, consider what will happen if interest rates go up to 8%; which is quite possible within the next 5 years.