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If i remember correctly it was used for hooking on datafactory events etc. It was used if you wanted to separate the code, like in a module and didn't want to use Global.asax.
Found an old blogpost by Allan regarding this with an example:
http://allantech.blogspot.in/2007/10/how-to-extend-general-page.html
I cannot really imagine well usage of PagePlugInAttribute Class.
What is some example of real usage for this?