I have indeed installed apps this way. I admit that most apps I've installed are from the market, although that's missing the point. Frequency doesn't matter

I have indeed installed apps this way. I admit that most apps I've installed are from the market, although that's missing the point. Frequency doesn't matter as much as the actual ability to make the choice independent of Google (or indeed any single corporation). If you think this doesn't matter in practice, consider Apple vs Google Voice on the iPhone. Apple rejected (or "didn't approve") voice, leaving iPhone users with no way to access it. Users would be better off if Google could have hosted a Google Voice package external to the app market.