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.
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.