I agree with everyone in here so far. We have mostly August, but then did a small (25 points) September add-on at one of the three resorts that we own. It is a separate member number and requires transfer from that account to the other if not used. MS has allowed me to combine points for a single reservation, but quite honestly I do not recall if that required an official transfer or not. OK ... just checked on the member website before I posted this ... it looks like it WAS an official transfer.
Bottomline, yes, it's a bit of a hassle. If I knew now what I knew then, I would have waited to do that 25 point add-on, but it was a fully loaded add-on, and I thought then it made sense. Maybe it still did, it's just that it takes a bit of thought and attention to make sure you don't make a costly mistake and lose some points.