I think the justification for this behavior is:
If you have an “older” project (say, VS2005), then no matter what version of VS you use to open the project, there will be a conversion process.
With this in mind, it makes sense to convert to the latest version. Converting a VS2005 project to VS2010 is not much more complicated than converting it to VS2008, but after the conversion is complete, you will have more options.
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