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I have a client who did this. He did it well and has hundreds of sites with these syndicated types of links from isnare, articlebase, thefreelibrary, article directories etc. Guess what? They got hit by penguin and panda. I can tell you it is of NO value. 90% of their back links are from these types of links and they are positions 200+ for the keywords that are in those articles.

It doesn't matter if the content is not spam or not. It could be the answer to cancer. Google doesn't know that, it's just going to see all these sites have the same content on it and as Matt Cutts said, they won't index all the other duplicates. And that is all they see, duplicate content. So they devalue it.

...maybe even penalize to stop people doing it.

Due to the way our system is and the way we want to do something. We have to make the description for each image in the ALT. Now this is not just a few words but is actually a few sentences. Is there going to be any negative disadvantage to doing it this way?

The positives I see is that it will help with accessibility and atleast the bots will be able to tell what the item is about.

The negatives is that maybe this description could be better used elsewhere?