Well, I'm a user, and when I see a crop tool, I expect something that crops, which is defined as "cut short", not hides with a mask. You are thinking like a developer - what would be easy to implement - and not like a user - what would I use it for. What you are describing is barely a small script addon, not a tool. Its functionality can be easily reproduced using the already existing tools: pages can be resized to a selection from the document properties dialog, so just create a rectangle in the desired size and press that button. Yes, you are saving some mouse clicks - but the crop you are suggesting won't be used more than once per document - once it's done, the document would become very difficult to edit. You are saving at most a few more clicks once every few days.
Now, that doesn't mean that I think Inkscape SHOULD have a crop tool. As you yourself probably think, a Boolean operation on each and every element that intersects the border might be slow, prone to errors and will destroy (i.e. convert to path) shape and text objects. And of course there is no way to crop filters... And most importantly I just don't see it as a very necessary tool.
Nevertheless, If we look at other vector apps, you will find that a crop tool has been implemented as a boolean operation tool - in Corel Draw: http://www.insidegraphics.com/corel_learning_tools/corel_crop_tool.asp
Illustrator Crop area tool does something else entirely - it defines export areas and does not hide or drop the area outside the crop during editing. http://www.adobe.com/designcenter/video_workshop/html/vid0213.html
If you want a page resize tool, just call it a page resize tool. you can add and optional "mask everything to page" check box in the dialog, but I promise you it won't see a wide use.