I'm sorry you've had mixed results with your recent order. I'm not sure why some switches would state that an upgrade is required and others wouldn't - you didn't mention what model you ordered.
Please understand here, if we shipped the "stable" code on new hardware, we'd never be able to get anything out the door on the same day the software released. As Brandon (and Ty) explained above, often, we must load "something" on the switch to make sure it boots so that we can begin the manufacturing process in time. Anything else would incur tremendous delays in getting our products out the door to customers like yourself. Unfortunately, development cycles rarely align in such a way to allow most companies to preload the GA release software on its new hardware. It's a common industry practice with many modern electronic devices - even Apple struggles with issues like this. I can promise you that we are testing the GA code until literally hours before it is released, much like many other companies (and even theatrical releases-- Star Wars ep7 isn't "finished" yet, but it has a release date). The required software has always been available on the day you receive your new equipment. Ideally, our customers are pre-staging new gear to give it a test and make sure it works as expected. Loading software should be a step in that final process before deployment.
Have you loaded any EXOS software on the X460-G2 or X450-G2 yet? The entire process from download, to install, to login after reboot takes less than 5 minutes! They're incredibly quick! I haven't seen firmware installation requiring 30 minutes of downtime. For the most part, the only downtime requirement is during a reboot, which shouldn't take very long. You can stagger this process across multiple devices in a network so that you can stagger your reboots responsibly and with minimal impact to your users.
I hope this helps explain why this process works this way and helps to bring some confidence in our methodology. We do it to help deliver the best products we can in the time allowed.