Batty,
Your quote"What rubbish...if the passengers expect the flight to last 2.45 and it takes 2.45 or less then the passengers are happy, they know what they are getting. So what if they say it takes 5 mins more than Easy the important fact is that the passengers KNOW what they are getting"
The passenger's expectation is not relevant in this case; on-time performance measures exactly that; the actual on-time performance. I agree entirely with the post by Befree; if an airline pads it's schedules by a greater amount then it's more likely to operate flights on-time(or within 15mins). That's simple maths.