Under Catalog > Categories > Display Settings > Display Mode there are three options - products, block, product & block.
I propose a fourth option for custom link, so users can link to a CMS Page or external link in the main navigation. There seems to be no easy way to do such a task currently.
Creating a block and then setting the category to block only, is not a proper solution and seem more like a hack. There are to issues with this method:
1. You cannot use it to link to external pages
2. Since there does not seem to be a CMS Page Suffix like categories and product pages have, any future change to the menu from a category block to CMS Page would cause a url change and affect SEO.
I see no reason why Magento should not support custom links in the navigation. My proposal above is one possible solution to the issue.
I sincerely hope this will someday be implemented. I was just about to add the same feature request. A way to add CMS pages to the top menu and external links is very important and we need full control of the order of menu items.
Therefore a way to create categories that redirect to a custom link is very important.