Temporary and permanent redirects are an essential part of how the web operates and are an essential part of white hat SEO in order to let the search engines know if a page has been moved or removed from the website completely.  When changing the structure of a website, valuable 'link juice' can be lost if pages with good inbound links are removed or their URLs are changed.  The search engine bots will follow these links from other websites and receive a 404 error (page not found) and all the value from the link will be lost leading to a fall in rankings.