And not just the consumers, but it would seem the producers too.

By choice, I do not have a Play Store account and wherever possible install apps only from F-Droid. However, there are (rare) times where I want to install a free (as in beer) closed source app. However, the publisher only chooses to make the app available via the Play Store. I would really, really like a direct apk download. I appreciate this approach is not for every end user and some people are scared of the "warning" messages Android displays when installing apps directly. Nonetheless, it would be really nice to have the choice and I don't see it as so much extra effort on the publisher's side.

Absolutely. If developers don't want one app store to dominate the marketplace they have to publish in multiple places. Otherwise what can they expect is going to happen? Don't put all your eggs in one basket.

My previous employer made it a condition of their BYOD mobile policy that I wouldn’t side load apps onto my device.

So even though it’s technically possible, it’s not always a going to happen.

There’s also side loading as a malware vector. If you do side load, you need to remember to turn off non-AppStore access immediately after.

"On Android you can easily sideload apps. So there is a choice."

If you develop apps to make money as a business this is not realistic. If the user can only side load your apps you probably have shrunken your potential customer base by 95%.

Managing app update becomes a problem outside Play store.