Dynamic HTML Landing Pages?

I am helping a construction company with some SEO. They want to have geographic landing pages as they've seen this work effectively for similar businesses.

Someone advised me that instead of creating individual pages for every geographical location, it would be better to create a single page running DHTML so that the page changes based on the user's IP address. All we want to change are the locations (city, region), nothing else.

What are the advantages/disadvantages of using this method compared to creating individual pages?