I've had steam on every computer that I've ever had since joining steam years ago. Not once have I had advertisements pop up due to steam, or any other program for that matter. Those that do and think it's because of steam need to seriously analyze what's really going on and scan for malware and viruses as steam does not inject advertising software onto your computer to make ads popup in your browser, other sites that you visit and programs that you download do that.
The world seems to be full of people ill informed and or people that just believe what Joe Montecarlo has to say on some anonymous forum somewhere about things like DRM and Steam and an assortment of other things. It's time to get informed people, it's time to get informed.
For those that have crappy ISP, you shouldn't be blaming steam for the way it works you should be contacting your ISP and complaining to them to quit knocking out your internet service on such a frequent basis if it's really happening all that often.
Our ISP generally does their maintenance in the middle of the night, like between 1 and 130am which for me is prime time so it bothers me, and I complain to them, not to steam or a game developer that uses steam. Now we have less maintenance, it's now been brought down to only once a week at most instead of the old schedule where it used to be almost nightly.