Mike,
Yep, I definitely understand and realize the SD file is uploaded to ArcGIS online when publishing from ArcGIS Desktop. However, once it's up there, there is nothing do with it. Yes, you have the feature service or map service to work with but once you make changes to that Feature Service it's no longer is the same as SD file the service is created from, right? What I'm saying here, is it would be nice to create another hosted service directly from that SD file without the need to publish it again from Desktop to get the original state back again. Sounds like ArcGIS Server Manager has this ability, to publish direct from SD file vs. from Desktop. Basically you could look at the SD file as a map template with all the fields you want to collect and then use it again to collect new features, you don't have to go back a publish again from ArcGIS desktop.