Some disadvantages of keeping documentation in attributes:
- poor formatting for long texts;
- no support by Visual Studio add-ons (e.g. using ReSharper's documentation preview feature);
- no support by 3rd party documentation generation tools;
- inclusion of documentation in assemblies which significantly eases reverse engineering;
- duplication of metadata in source codes with metadata stored in a version control system (there's no point in tracking any declaration's author and version in the source code, when the VCS gives you much more precise information — VCS's don't lie).
I can't think of any advantage right now. In case I would really need it, it's always possible to parse the XML documentation comments and transform the whole codebase into any attributed form.
