AstroCat, you are fighting against the orthodoxy of this board, because many of the members who are arguing against Steam don't like the service at all to begin with, so they would be very hard to convince that having that platform selling this game would be of benefit. For example, Kayoz was very critical of Steam as a platform in the past, and he is well within his rights to do so.
Some also mistake Steam and Steamworks, which are two related, but different things (one is the store where people want to buy the game instead of Matrix so that it is in their library), the other is if every copy of the game required Steam, which I don't think anyone is really arguing for).
I don't think anyone is saying that everyone should be forced to use Steam or any other digital platform. I think people just would like the option.
I used to be very much like Kayoz, and it was just a few years ago that I would have totally been on his side about having discs and the original files... and then I started using Steam a bit more and I've totally changed my mind about that particular service because it makes it easier to manage my library of games, expansions and DLC.
I don't know if Matrix is still selling their games in brick and mortar retail outlets, but if I am to believe Eurogamer about the percentage of profits that publishers generally get from selling in those locations (30% but that is for console titles), that seems like a much lower figure than the much bandied about ~70% that the publisher gets from Steam. If I took the LA Times/OnLive's word for it, the publisher is pulling in $27 from a $60 boxed retail game or 45%.
And I think the reason why people keep bringing up the impact on sales that just being on Steam has is due to the fact that there are many, many stories about sales figures for games that include numerical data. I am going to start with two for now since this is getting to be a long post for this thread. For instance, when you look at the first 6 months of sales for The Witcher 2, it sold 240,000 copies. 195K of those sales were from Steam. The other 45K were from GoG.com (which is run by the company that released the game and sold 35K of that remainder) and the other digital distribution sites, all of whom were selling it DRM-free. When given the choice, 81% of people chose to buy it from Steam over a DRM-free version, which in GoG.com's case was even more egregious because their version had additional content that the Steam version still does not even at this moment.
Or what about the story of Chris Park of Arcen Games talking about having AI War 2.0 picked up by Steam and Direct2Drive in October 2009 and that literally increased the revenues that the company made for the entire year by nearly 5 fold and allowed both him and Pablo Vega to quit their jobs and make games full time. And just last year he said that his company would not be able to survive if it wasn't for Steam.
AI War Fleet Command is a niche space title that Matrix distributes as well that isn't just doing a little bit of business... it is doing extremely well because of Steam. And given the fact that the revenues that Arcen is making on that game (especially those generated by the massive sales on the title they regularly have) is not only fueling development of additional content for that title but also partially funding new projects, why wouldn't someone who sees that happening for one developer in the Matrix stable, especially after you've also seen the initial sales figures for competing 4X games that were released relatively recently.
Of course, I am pretty sure that everything that I've just said has been said elsewhere on this forum so I am going to stop for the moment.