Google, yahoo and MSN search engines had been frowning on affiliate URLs for some time. Lot of affiliate sites got booted out of search engines over a period of time. Affiliate Urls are itself not a problem unless the site runs by itself and doesn't depend on affiliate Urls and affiliate income. Any good quality site can have affiliate URLs or any ads like adsense etc.
But affiliate only sites and MFA ( Made for Adsense ) sites don't work anymore. SO if you are planning to start sites like these we recommend not to do it.

SEO Blog team.