You know, if Valve (or anyone else) didn't have that data I could kind of agree on that point in a sort of idealic way. All that should matter is if you're doing well and making a profit. Who cares if X game sold that much more. It's that kind of thinking that supposedly drove Bioware to make Dragon Age 2 what it was as they saw other blockbusters selling millions more. Of course, I know nothing exist in a vacuum and people will choose to buy one game over another because of available funds and these charts help measure success in a certain respect. I sort of think that Valve doesn't produce enough games to be considered competition for a lot of games out there and releasing sales data to the public really doesn't do all the much useful, but I'm sure publishers and developers could benefit from seeing more. It would probably help the arguments against the PC as a viable platform and it would help them judge the potential market size for a given product and, by extension, a budget.
Of course, I'm sure Valve would be more than happy to release the data for a price.