Interesting topic and worthwhile to bring it up. I suspect if someone tries to hang some google adwords banners, then they might not get upvote love in the near term. Perhaps we'll see a trend where the ads are placed into a post just before the editing period is over and most of the upvoted is said and done for the first 24 hours. I posted a video to youtube and did not want the ads to run within my post so I left it turned off... which reminds me, I ought to go get that rolling.
My guess is that a small amount of adverts - like one per post would be tolerated by most. At some point, if they stop working - the broken links will start to become an issue as will image links assuming they are only cached locally.
A third party Steemit advertising initiative has been set up. The first of many perhaps.
https://steemit.com/steem/@condra/why-i-m-in-favor-of-advertising-on-steemit-better-content-and-passive-income