What happens if user supplies two directories on command line, where both contain a project?
IMHO opening a project just by specifying it's directory might be confusing. If I'd want to open a project, I'd pass it's project file as argument (which already works well). As for passing directory on command line, much more intuitive behaviour (again just my own opinion) would be to open all files in the directory. Creating a new project could be quite a surprise, especially for users who don't usually use projects.