It may be a convenience that many enjoy, myself included, but I'd argue that plenty of Netflix subscribers would have subscribed anyway even if ads were shown from the start. Personally, I signed up for the convenience of movies, so ads weren't even a consideration. I doubt I'm alone in that respect.

In addition, you can easily get the same ad-free content on iTunes, but I never see it compared as a replacement for Netflix. It's just not as convenient.