First, There is a definite reason for these urls in magento. URLs like
site.com/catalog/product/view/id/2609 are used by magento's controller-action philosophy so that it can provide correct response to the request, as URLs like
site.com/some-real-friendly-product-name don't provide anything to magento to know what exactly is requested. You can read more about this approach in this article from magento knowledgebase.
The reason for converting URLs with ID to friendly URLs is obvious, SEO, As search engines prefer human friendly URL structure.
And there is no downfall for providing 301 redirects as they tell search engines simply that the page requested has been permanently shifted to another URL so that search engine do not consider them as two seperate URLs for one page(duplicate content issue). So, don't worry about them at all, they are not gonna harm in any manner.