When they go down because of the program’s servers, you alienate visitors, your site looks uncared for and outdated.
Aside from any SEO benefits, that’s why it’s important to use the img alt tag, visitors that may have images disabled or are on text mode only browsers can see what the banner ad is about. Nowadays, surfers seldom have graphics disabled and hardly any use text only browsers anymore except for accessibility users.
When a page loads using Opera you can see the load speed time, number of images and each remote path the page must take to serve it’s contents.
You’ll be able to see which server is slow in responding.