The meta:progid attribute on the Page directive is necessary if the page will ever be opened in SharePoint Designer. You should include this as site pages are supposed to support customization and thus, be opened in SharePoint Designer. If you don't want to allow pages to be opened and customized in SharePoint Designer, then you should do that through permissions, not at the page level.
Using ASP.NET 2.0 Code Behind Files in SharePoint v3 Sites
SPFile.ProgID gets a string that identifies the application in which the file was created.
That's all the meta:progid is about. It's a identifier for SharePoint and you probably should not use other attributes, they might cause run-time error.