Whether google can index the feeds is very unrelevant.
What you are doing though is providing additional content for your users as well as creating another inbound link from a website. ( More relevancy)
Google loves anything you do for your users.
Image alt tags need to be used for vision impaired readers. They use text to speach programs and if your image tags are blank, = low rank.
Also unless you use a robots.txt file to keep the bots out of areas, they will spider your entire server pages. This includes previous html copies left lying on the server, which in many ways can be adverse.
Sergy & Brin are very much into accessibility for all users, sort of their activism stance and as such people need to realize that the smallest things to website owners are the biggest thing to google.
By the way can you get me Prince tickets for the Philly Show ?? lol