Is it always bad practice for a single web site to have 2 different versions of the main navigation depending which page you are on?

I oversee a large site that consists of several product pages along with lots and lots of SEO material. To keep our page rank high, we have a navigation designed for search bots but which is difficult for humans to use.

I'm evaluating doing something I normally consider unacceptable: using one version of the nav on the SEO pages and another (simplified) version on the product pages.

What are your thoughts about this? Have you seen this done before in a way that was actually good and not terrible?