While this block on side-loading is GREAT for blocking paid apps being bootlegged, it's a pain for developers as well.
Because, to test beta versions of programs, we have to load the APK file. And, you find that apps work on rooted phones and then break on the non-rooted ones. So, for beta testing, you need to do it on Non-rooted phones. For example, I've been a beta tester for a long time of a program called "CacheSense" on the BlackBerry platform. Best Geocaching program out there, and even as a Beta tester, I still paid Bruce (The developer) for the app.
Now, he's working on an Android version. I want to beta test, but I can't side-load without rooting, and that makes my results void.
I've been using the "Sideload Wonder Machine" to side-load apps on my Atrix for free without rooting it.
That being said, I never, ever, ever, ever, ever, condone side-loading to load a paid app for free. That's just stealing from the developer. If you really want a paid app, pay for it. That encourages the developers to improve upon it and release newer, better versions.