You might worry that you wouldn't notice which sites are using it. Or you might want to send a message to sites, or to Mozilla, that you think DRM is bad and that you don't want to have DRM implementations installed.
One of the worst things about each browser vendor's decision to support DRM is that it makes the choice to require DRM less costly for new web sites that are considering it. So even if you think Netflix could never have been budged, random site X might now say "cool, we can control what people can do with video on our site, at a surprisingly low interoperability cost to us!". If you want to avoid that outcome as an end-user, you have to do whatever you can to increase potential DRM adopters' view of the market share they will put at risk.
Also, if you do visit a site that uses DRM and your browser doesn't support it, it will show up in the server logs, so just using it is sending a message.
This end does not seem to be served by niche browsers which will be used only by the EFF-card-carrying subset of the tech community, people who are most able and least ethically inhibited from piracy anyway.
This is a good reason to keep the functionality out of mainstream browsers, but it doesn't seem to work if it's just us using deliberately hobbled browsers.
How typical of the DRM/closed-web subset of the tech community to justify their collaboration with accusations of piracy at those of us trying to create a free and open web.
Resisting any type of DRM from the very beginning is important, because the fight will be a lot harder once there is an established community of users. Educating people about DRM is hard enough; it is almost impossible when it also requires convincing people to give up some service (i.e. netflix) that they have grown accustomed to.
This is the latest battle the ongoing War On General Purpose Computing[1], and a lot of people that should know better are choosing movies and the dream of short-term profits over long-term freedom.
One of the worst things about each browser vendor's decision to support DRM is that it makes the choice to require DRM less costly for new web sites that are considering it. So even if you think Netflix could never have been budged, random site X might now say "cool, we can control what people can do with video on our site, at a surprisingly low interoperability cost to us!". If you want to avoid that outcome as an end-user, you have to do whatever you can to increase potential DRM adopters' view of the market share they will put at risk.