Bristol_Jonesey
There is no practical reason to export & re-import.
Yes there is and the most practical reason is after mixing for a long period of time you are no shape to master. Your ears are fried already. You will make very poor mastering decisions.
What you do in mastering to an overall mix is quite different to what you are doing in mixing. You certainly can apply all the mastering processes to a mix while you are mixing but the best mastering engineers are actually not doing this. And they usually ask the client either to not do anything to the mix or very slight compression is considered OK.
You should aim to get the mix sounding as great as you can without anything on the master buss.
(slight compression is actually OK and can give you an idea of what a compressed mix will sound like.) Some say the compression on a master buss can change the mix slightly. I have found from experience if the mix is really great then this applies even less.
Print your mix
(without mastering applied at the time) and live with it for a week. You will be amazed at what you hear and I can guarantee you will refine it and modify it for the better. Even after that, leave it for more time and approach mastering fresh.
(In the morning too after a good nights sleep) And by the way when you start mastering this way the decisions you make will be totally different to the decisions you make if you master at the time of the mix.
Great mastering is a slower process. Let go of trying to get it all done in record time.