I already have Arma X and don't need a second copy. The digital soundtrack and the map are nice but I prefer physical extras (like the printed maps you can buy at the BI Store). Maybe some of these things will be included as physical extras in the box version.
So whats left of the Supporter Edition? The future DLCs? First they have to release the full game. Then it took over a year before they released the first Arma 2 DLC (BAF). That's why I don't expect to see any Arma 3 DLC until next year and for me the inclusion of future DLCs are not a selling point.
For me theres only one interesting point left: the end credits. I've backed several games on Kickstarter and in each game the developers offered a special package where your name was put into the end credits. I know, for most people this is really unimportant and I guess only a few will read through the end credits. But for some of us it is really important. And no, it's not about fame (as I said, probably no one will read these credits) or to brag about it. I think the main reason is that you can be a part of one of your favourite game series - even if it's only your name in a wall of text.
It's really clever because a lot of people would willingly spend additional money for this and it would cost the developers no money to include all these names. That's why I don't understand BI. What's the point for limiting this extra to the first 500 buyers? Let's say two people buy the supporter edition. Both spend the same amount of money and support the developers but in return BI only thanks one of them and put his name into the end credits because he clicked the Buy-button 5 seconds earlier. In my opinion thats really unfair. I know, the supporter edition is about supporting BI and paying a bit more without any real benefits (especially if you already own Arma X) but as I said it would cost them nothing to include all the supporter names (or at least the names of all the people who bought the supporter edition during the first day or maybe week) and not only the first 500. A small "Thank you" where both sides would be happy. I would have bought the supporter edition but I changed my mind because of this. So to get back to the topic: I will only buy the Alpha Version.