Drew Olanoff, regarding the removal of background images from Twitter’s home and notification pages:

The reason? None officially given. One of the possibilities, and something I’ve heard, is that Twitter wants more control of their ad display experience. For example, if the company were to want to sell a full-homepage-takeover-background-ad that you see on a lot of sites, they couldn’t. Because of your cruddy customized one.

I’m glad I only interact with Twitter through Tweetbot. The interface is clean and reliable.