You are talking about sideloading in Windows 8/RT, not Windows Phone 8. You can also sideload third party apps without a sideloading activation key in Win8, it

You are talking about sideloading in Windows 8/RT, not Windows Phone 8. You can also sideload third party apps without a sideloading activation key in Win8, it's how testing on devices is done. You get a developer certificate for yourself (completely free and one button press when you first try to sideload) and essentially self-sign the apps. It's not as simple as double clicking, but it's one Powershell command (Add-AppXPackage IIRC). It's much easier to sideload with the sideload activation key, yes, because sideloading as a whole seems to have been created for Enterprise App deployments, but it's fully possible without it. I believe the only restriction is you need to renew the developer certificate every few months.