Completely agree with the point that you can accelerate audience building on a major platform like this one, but in the end, you really don't "own" anything. Guest posts are great for building credibility and backlinks, networking, etc. At the end of the day, if you are blogging to support a business, you still need your own web presence.
For individuals, though, I mostly agree with you. I have a blog that is "just for fun", and it doesn't merit the time and effort required to push it up the SERPs. In retrospect, I would have probably been better off doing it on an established domain.