There is confusion between attaching a (static) file to something (such as a calendar entry or an e-mail) and linking to a changeable file. Apple provided an attachment feature, not a linking feature. Each has advantages and disadvantages. I believe that iCal should offer both (with clear explanations of the differences).

iCal does offer both.

If you drag a file to the attachments… link, then you copy an attachment.

If you drag a file to the url field, then it will place a link to that file there.

The only annoyance is that you can only have one url per iCal entry. (But you can link to a folder, which may have several documents).

I personally use this feature all of the time. Work I have to bring to a class gets a link in the iCal entry for that session, so I can quickly load it up at class.

A URL link is not a full-featured link. A URL link breaks if you move or rename the file. A true link can handle name and location changes.