MOSS Crawled Properties

While discussing about "Crawled properties" I came across with one behavior. I have framed it as a questions & answer format to understant it easily.
Question: Why the check box "Delete all unmapped crawled properties" under edit category doesn't work properly? Is this behavior by design? If yes justify? Also a point to notice that all unmapped crawled properties under edit category should get deleted when we check box "Delete all unmapped crawled properties".

Answer:
We will not be able delete "SharePoint" property names if they are being used by SharePoint. It is by design behavior.
There are two conditions where we can’t remove "SharePoint property" if the :
a) Property is being used by SharePoint. [system property] and/or
b) Property is "Included in index".

STEPS TO REPRO:
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1. Navigate to Shared Services Administration from Central Administration
2. Click on Search Settings
3. Click on Metadata property settings
4. Click on Crawled Properties
5. Click on the SharePoint category an note some of the crawled properties without any corresponding mapped managed property. For example "ows_BaseName(Text)"
6. Click on Edit Category
7. Check the "Delete all unmapped crawled properties" click OK
8. Observe that "ows_BaseName(Text) and other crawled properties (that have no mapped managed property) have not been deleted.

for more information:
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http://msdn.microsoft.com/enus/library/microsoft.office.server.search.administration.category_members.aspx
http://msdn.microsoft.com/enus/library/microsoft.office.server.search.administration.category.deleteunmappedproperties.aspx