Understood, but points are fungible. You would transfer the 500,000 you have to BA (for example) and then you would buy what you need for the next redemption. The outcome is exactly the same. Identical amount of money spent, same number of points redeemed.
I just don't see where it says (in the T & C's) you have to run your balance down to zero. As a matter of fact about 3 or 4 years ago I was able to buy 500,000 MR points when I had a balance of a couple million MR points. Again, I think you are assuming nobody would want to buy MR points if they already have enough to cover the transfer. I do.