What's the use of Server, X-Powered-By and other similar headers? Looks like the consensus is they should be removed so that automatic vulnerability scanners doen't immediately know which version of which software they're dealing with and so automatic vulnerability discovery gets harder.
Are there scenarios where it is indeed useful to let all the world know that the site is running on IIS 7 and is X-Powered-By ASP.NET version 4?
It might be useful to developers or Power User clients; but is certainly more commonly useful to hackers and their like. I would leave them on while developing a site, and perhaps upon initial release; but not in the long run. Clients care about content, not headers.