IIS is unique in the sense that the version that ships with an operating system is the only version available for that operating system. Windows 2000 Server got IIS 5, 2003 got IIS 6, 2008 got IIS 7 and 2008 R2 has 7.5.
While they are version-locked, there are very significant topical differences between the two tags. To people who only care about the web-server, following the windows-server-2008-r2 tag will yield quite a bit of stuff they don't give a hoot about and may not be able to help with. By having a focused tag on iis7.5 those people who are interested in web-server config, but not user VSS shenanigans, will be able to follow a much more relevant tag.
I believe this is one case where a focused tag is appropriate.