Google could, at any time, initiate a cloud-to-device intent to install arbitrary software that you "wanted" from Google Play. They don't unless they ask you to, but the capability is there.
The bigger thorn is if you're on a device where the manufacturer or carrier has hidden or removed the option to install arbitrary APKs.
When I interned at Facebook two years ago, it was a PITA to change the API to work around bugs that were fixed in old versions of FB4A because people wouldn't upgrade -- whereas the vast majority of iOS users upgraded the Facebook app within days of a new release showing up in the App Store. The addition of non-Play devices makes this even worse.
Granted, I had my own reasons for not wanting to upgrade, so I can sympathize with users, too. (Facebook was pre-installed on my device, and upgrading caused it to take up space in the 180 MB volatile/userdata partition on my Nexus One. I eventually solved the issue by buying a Galaxy Nexus, but I recognize not everyone can buy new phones that frequently.)