From the API proposal doc:
Additionally, we’ve seen numerous extensions create sidebars by injecting HTML content into the current page. It is likely that the majority of sidebar extensions would use a native sidebar if available. Here’s a sample of some of our favorite sidebar extensions...
Some of these tools charge over $5,000/yr/seat and have hundreds of large corporations like Facebook, eBay, Salesforce, Pinterest, Twitter, and Netflix using their software, which demonstrates the immense value that can be provided by a sidebar.
But that's because up until now we couldn't care about it anyway if Chrome didn't support it. Sidebars can easily make a comeback if the functionality proves to be useful, and I can picture many extension developers experimenting with it if it's made available.