I think the CAA are covering their backside slightly. They offer us what would happen in the worst case scenario, and if you're still good to go then fine, go.
However they may think that some pilots would become over confident with proverse factors and do precise calculations if they're in a tight situation, which they then work out wrongly, and get stuck in a hedge.
There have been a few times when officially i shouldn't have gone, but i knew the a/c and the conditions were favourable and made it easily. Then again there are also times when according to the book i could go, but i knew the engine(s) were old and the a/c was a dog to fly so i jacked it in for that day.
I agree that proverse factors can be useful, but then there needs to be a simple way to work them into the other calculations (remember we're only pilots, not mathematicians
) i still remeber K.I.S.S. from flight school.