You can keep a room-only reservation and buy the tickets separately so it isn't a package...but not the dining plan. If you want a dining plan you don't have a choice but to eventually book a package.
That is what we did for our vacation last month. I booked a room only in July. When a RO discount was announced in October, I called and got that applied to the room. Then at the end of December, when the discount period was due to end, i called to make sure that I would not loose my room discount if I added the dining plan in January, which was after the discount had expired. I was assured that I would not loose my discount.

So then I told the CM that as long I was on the phone with her, I might as well buy our tickets. So I bought the tickets in December, had to pay for them in full of course, but didn't have to pay the balance of the room yet.
Then I called about eight days before our trip began to add the dining plan. The very first thing that the CM said to me was that she saw that I had a really good discounted rate for the room and that she was going to be sure to keep that for me.

Then she cancelled our original reservation, refunded everything we had paid to date, which was the one night deposit and the price of the tickets, and gave us a new reservation number. We had to pay in full at that point since we were within the 45 day period for a package. There was no mention of whether or not my room was still available for the discount. I was already told that I wasn't going to lose it.
I had waited until my credit card billing period had rolled over so that the refund and the new charge were all in the same billing period. It took a week or more for the refund to show up in my account. It all worked out very well.
As long as there isn't
free dining offered, I can't see any reason to buy it ahead of time.

I was able to keep my discounted room and not have to pay for the dining plan until after my trip was over.

ETA: At the time I took my vacation, which was Feb 5-11, 2013, the only two discounts available were the stay, play and dine plan, or the room only discount. Since I saved more money with a room only discount, I was able to add the tickets and dining plan later. If I had wanted the stay, play and dine plan then I would have had to book the package from the very beginning.