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I bought the Guild Wars 2 prepurchase yesterday knowing that the game was not completely finished and the cash shop items had not been finalized... and I thought I'd share why I've done this despite never having ever preordered a game before.
How many MMOs change after you purchase them? All of them do, and most change in very drastic ways over the years. Buying a finished MMO doesn't guarantee that the game won't change for the worse at any given time.
How many cash shops change over time? I'm not aware of any game with a cash shop that doesn't add, remove or modify the items that they sell through the cash shop over time.
Given the numerous Guild Wars 2 beta videos that are out there I am confident that Guild Wars 2 is not just playable but has more polish than some recent final releases have had despite being incomplete. I'm not afraid of a Vanguard repeat. Even more, since Guild Wars 2 has no monthly fee the only way for them to keep making money off the game is to make the game so fun that I want to keep coming back to buy expansions and cash shop items. This isn't a problem for me.
Given Arenanet's reputation with Guild Wars and knowing that players can trade in-game gold for cash shop gems, I am not concerned with future cash shop items. If the cash shop items ruin the game then I'll stop playing... until the problem is addressed. The game has no monthly fee. I'm not out anything if I decide to play some other game. It's not like I'll have a subscription burning while I stew over unpopular changes to the game. This isn't a problem for me.
If you're not sure about the game or the cash shop then don't prepurchase it. It's not like they're going to run out of digital copies after the game is released. Wait and see. I'm sure that other people who have prepurchased, like me, will be posting game reviews after each beta weekend all the way up to and after the game's release.
I don't think that prepurchasing an unfinished game usually makes sense but it did for me this time. I'd probably prepurchase diablo 3 (in fact, didn't people already do this by signing up for blizzard's annual pass thing?) since blizzard is fairly reputable, but I can't think of any other games that I'd do this for off the top of my head.