Firstly, I'm sorry to hear that you might have gotten some misinformation during your in-store experience. Viewing your streaming content via the Roku definitely requires the SlingPlayer application for whatever mobile device you are casting from. The upside is that once you purchase application for your Android device, you can install that application on an unlimited amount of other Android devices (as long as you can access the same Google Play account). The only limitation to that is that if you purchase the Android tablet version of the application, it won't install on the Android phone. However, you can install the Android phone application on an Android tablet.
If you are seeking an alternative option to casting from the Android device to the Roku, I would suggest the Amazon FireTV. It's about the same cost as the Roku, but doesn't require the secondary mobile device to view your Slingbox content. You can view your Slingbox directly through the application on the FireTV.
Sling Media Technical Support