Duplicating the categoryid over and over just isn't very logical. As it becomes a bit redundant. Granted I've done this in the past to give clients 10-20 more categories but its not "unlimited".
So my theory was why not use just one category field in the products table with a delimeter and id's of the categories to associate the product with, then explode the new category field upon searchs, etc.
Just my 2cents.
btw, one thing I hated about the modify product search is its in-ability to search any other category but the main category. I've got a mod for this if anyone is interested PM me.