Thanks for your feedback.
Indeed Bower does not provide the latest available version, but the latest non-beta one
You can check that by installing ionic via Bower CLI
Maybe you were misled by our details pane
for the package which we will try to improve in the near future.
The ability to choose the version is a nice feature and a great addition. That is why I would suggest posting a feature request at our feedback portal
where our community could vote for it and raise its priority. Also you can easily track its progress.