This is a good question. I would say that stereotypes in advertisements are powerful in shaping culture. For this reason, advertiser have a very big responsibility, which often times they forget. Let me explain.
First, advertising is ubiquitous in our world. We are literally bombarded with advertisements from the moment we wake up. We see thousands of advertisements all throughout the day - radio, TV, magazines, Internet, billboards, strategic product placements, and so much more.
Second, advertisement shapes culture and it preys on people's desires. So, if advertisements reinforce stereotypes, then people's worldviews will perpetuate more stereotypes. This can cause problems in our society, because people will perpetuate ignorance.