I don't think its illegal to not have it in the plane( I never do) but the point is you have to check it to see if it contains any mistakes before you fly,and the only way to do that is by having it in your hand.
If you fly without having it and have an accident and it susequently turns up with mistakes you will not have been insured at the time of the accident as the onus is on you to check that you are legal before you fly.
Of course you wait in lovely weather only for it to arrive in perfect condition and then the weather craps out for 6 weeks!
