I know this is an important topic to a lot of people (and it likely reflects the same policy that will be utilized for Kingdoms of Amalur: Reckoning), so I'm going to point you over to a newly posted Q&A from BioWare's Chris Priestly that confirms Mass Effect 3's Origin requirement and what it means for your day-to-day gaming entertainment. The lack of Steam availability is a pretty serious issue for me, so hopefully Valve and EA come up with some kind of resolution soon:
Below are answers to the most commonly asked questions about Origin and Mass Effect 3. Thank you for your patience as we have worked to answer them.
1) Will Origin be a requirement to play all versions of Mass Effect 3? (Digital and/or from a retail brick and mortar store)
Yes, Origin is required for all PC editions of Mass Effect 3, physical or digital.
2) Is constant Origin connection required or is it a single one off authentication when the game is first installed. Is there also a limit to the number of installations available?
Mass Effect 3 will require a one time, single authorization for the single player game. There is no limit to the number of installs. Playing Mass Effect 3 Multiplayer will require a constant connection.
3) Is Origin required for the retail versions of the game?
Origin is required for the PC versions of Mass Effect 3, both physical and digital.
4) Will ME3 be available on Steam?
During initial release Mass Effect 3 will be available on Origin and a number of other 3rd party digital retailers, but not on Steam at this time. Steam has adopted a set of restrictive terms of service which limit how developers interact with customers to deliver patches and other downloadable content. We are intent on providing Mass Effect to players with the best possible experience no matter where they purchase or play their game, and are happy to partner with any download service that does not restrict our ability to connect directly with our consumers.
5) Is there an opt in or opt out clause for data collection?
Users will be allowed to opt-out of Mass Effect 3 data collection from inside the game.
6) I’ve seen reports that Origin is spyware. Is this true?
Origin is not spyware, and does not use or install spyware on user’s machines. In order to allow Origin to install games and their patches for everyone to use, Origin implements a permission change that results in Windows, not Origin, reviewing the filenames in the ProgramData/Origin folder. This is an ordinary Windows function, not an information-gathering process.
7) Has the Origin EULA been updated following recent questions in Germany?
The German Origin EULA has been updated. For more information, please review the Origin EULA here:
http://www.ea.com/1/product-eulas.
8) Is there a double-opt in for Mass Effect 3 marketing emails for German residents?
Yes, we always adhere to the German requirement of a double opt-in standard for marketing emails. Mass Effect 3 users must opt-in twice to receive these emails.
