Anyone who wants to install a non-Play Store app, can. If you're someone looking at a piece of software on the internet, the application's website can easily offer you instructions on how to download and install their software. This is exactly how Windows has always operated.
A perfect example of this working well on Android is the Amazon Appstore. Disregarding for a second the meta-ness of it being another app market, they do a great job of explaining to the average user how to download their software (by pushing it to them through email, text, etc.), "allow non-Market applications" and install the APK. Sure, not every user is capable of doing this, but I think anyone who has helped non-technical family can attest to the fact that the same is true on Windows.