As long as it isn't trivial for the casual user, I wouldn't call it much of an issue. As I said, dedicated pirates will find a way regardless, even if comes down to screen capturing. That's how people are pirating Netflix content at the moment. Of course, this does lead to some level of quality loss, since you're doing a lossy re-encode of a lossy source.
On that note, if we consider "wannabe pirates must re-encode the video to share it around" as a decent video content protection goal, then you can definitely do that with HTML5 video and some JS without resorting to any kind of EME trickery.
Anyway, at the end of the day, it is definitely true that JS content protection schemes will be inherently weaker compared to black box DRM plugin solutions enabled by EME. But why on Earth should we compromise the very nature of Open Web to enable this rather than make Big Media compromise on their platform control addiction to have their content on the Open Web? And if they're unwilling to do that, then well, they can stick to their Flash and Silverlight all they want in my opinion. EME doesn't make any promises about cross-platform compatibility anyway, so better stick with the two devils we know than switch to a system comprised of several unknown demons.