There are a few elements to it that one could use to dispute its value — for example, Netflix has no way of knowing what level of service a customer has paid for. They also don't appear to differentiate between different types of service from a single ISP (e.g., they lump together AT&T's old ADSL customers in with ADSL2/U-Verse, and, presumably Verizon's DSL with FiOS). I would hope that they are also filtering for sessions where a higher bitrate is requested, which Netflix's Silverlight app doesn't do unless the window/screen is large enough to benefit from the extra resolution.
Of course, one could argue that those differentiators don't ultimately matter, and that if its customers are opting for older or slower service levels, that can tell you something about the ISP's pricing competitiveness.