Richard M Stallman (RMS), the father of free software movement has spoken about Steam's arrival on GNU/Linux systems.

Stallman has said what was expected from him; while he admits that games like Steam will make GNU/Linux popular he also warned about such games.

RMS says:

I suppose that availability of popular nonfree programs on GNU/Linux can boost adoption of the system. However, our goal goes beyond making this system a “success”; its purpose is to bring freedom to the users. Thus, the question is how this development affects users' freedom.

I don't think RMS doesn't want GNU/Linux to be a success, or become popular. I have met RMS couple of times and what he does want is 'user's freedom' to become a success; he wants a user's freedom to become popular. It's similar to making 'healthy food' popular and not just make 'food' popular.

There is no question about why RMS is critical of non-free software whether it be an OS like Windows or games. RMS writes:

Nonfree game programs (like other nonfree programs) are unethical because they deny freedom to their users. (Game art is a different issue, because it isn't software.) If you want freedom, one requisite for it is not having nonfree programs on your computer. That much is clear.

I do feel that there will always be the non-free gaming market due to the business model they have. So, in a case like this it's better to play such games on a free GNU/Linux system than on Windows system. At least you are running it on a free base.

However, if you're going to use these games, you're better off using them on GNU/Linux rather than on Microsoft Windows. At least you avoid the harm to your freedom that Windows would do.

RMS, rationally, believes that arrival of Steam on GNU/Linux can do both harm and good:

Thus, in direct practical terms, this development can do both harm and good. It might encourage GNU/Linux users to install these games, and it might encourage users of the games to replace Windows with GNU/Linux. My guess is that the direct good effect will be bigger than the direct harm. But there is also an indirect effect: what does the use of these games teach people in our community?

But then he argues the damaging effect of non-free software:

Any GNU/Linux distro that comes with software to offer these games will teach users that the point is not freedom. Nonfree software in GNU/Linux distros already works against the goal of freedom. Adding these games to a distro would augment that effect.

If you want to promote freedom, please take care not to talk about the availability of these games on GNU/Linux as support for our cause. Instead you could tell people about the Liberated Pixel Cup free game contest, the Free Game Dev Forum, and the LibrePlanet Gaming Collective's free gaming night.

I agree with RMS, as always. I believe Steam might give users more reasons to use GNU/Linux. At the same time this success with encourage more players to adopt GNU/Linux. Even if they will use one non-free component, Steam, on their GNU/Linux systems they will be using the rest of the free software stack. The increasing popularity of GNU/Linux will encourage free software developers to enhance the quality of their applications as they now have a bigger user-base.

It's hypothetical but the success of free software base may encourage players like Valve to eventually find a different business model and release their software as free software.