The images provide a standard look and feel to the action bar which will make it consistent across all platforms. The assets are pulled directly from the Android 4.0/4.0.3 source so it will look like the Holo action bar from ICS.
The themes will not affect any other styling of your application widgets so buttons will still look like the native buttons and text fields will still look like native text fields. Only the action bar will be themed. I do not advocate backporting the entire Holo theme to use in your application because it will only create an unnatural experience for the user.
One thing you can do, however, is take the styles for buttons and text fields from ICS and customize them a bit to create a branding of sorts for your application. This way you are afforded the clean and polished look of ICS but with a unique twist that will blend in seamlessly with your applications color scheme and feel.
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Jake Wharton
http://about.me/jakewharton