If a monitor doesn't support HDCP, one of two things will happen at the discretion of the content providers. It's a possibility that a given studio may simply refuse to allow the content to be displayed at all. More likely, the studios will allow for playback on unauthenticated devices with purposely degraded quality. The thinking is that Joe Consumer will be more likely to pay for HD content than seek out pirated content that's not in HD
Essentially, this is a sneaky way of forced upgrading, whether or not you actually need it. Want to rent DVDs with HDCP video? Better get that newer screen!