I don't think it's the DRM that facilitate anything. DRM is a tradeoff between pirated stuff and going to the theater. Netflix is a legal Napster while Hollywood are content with DRM as good enough protection.
I require a definition of how they are "considered harmful" because I seriously doubt that.
What's to stop someone from creating their own web client that doesn't implement that part of the standard? Where's it written that we have to follow every bit of the standard as it is written? Who gave this select group of people that much power over the open web?
Personally, I think the W3C was approaching being corrupted long before this issue. Maybe not in the terms you are describing right now but I've had issues with the behavior of various standard bodies and browser providers that happen to be a part of these bodies for a while now.
As for artifacts of culture being locked away from future generations, that is a potential problem. But we all know a silly DRM scheme isn't going to stop that. But that kind of thing has been in practice for a long time, way before we had electronics and the human race seems to be moving along in despite.
To me, a better solution is to show that people are willing to move on from these aspects of culture that everyone is so hung up on. If enough people show that these schemes don't work in the long run the content providers are more apt to change their ways. Look at the music industry, it's vastly different than it was just a few years ago and continues to change. Let these people attempt their schemes, in the long run it will matter little.
I keep pointing out that people are claiming that this is harmful in some personal way. I keep asking how. The best I've seen is that it creates harm to the standards, which is more about causing harm to the people who control the standards. This decision directly harms the standards body itself, it does not harm the open web. Corrupting the standards with bad decisions hurts the reputation of the standards body, which will eventually importance in the scheme of things. The open web will just move on to something else. The reason I say that is because if the web is truly open, it is free to ignore the "standards" it doesn't like. Meaning, the people who use it can choose to not participate. It's a standard because everyone agrees to use it together. If the people using the open web decide they don't like a browser that implements DRM they don't agree with, then they have the option to choose a browser that doesn't implement the DRM. If there isn't such a browser, the open web allows someone to create and share it with the masses.
If this decision actually causes harm to the open web, then it's not truly open because that implies no one can get around the decisions made by this select group of people. If that's the case, then the web doesn't sound very open as it stands today.