I think that these advertisements should run only once or twice a week, at most. I feel that the pop up displaying along with every virus definition update is a bit excessive.
Also, I feel like switching antivirus programs isn't a good solution, as you're just sacrificing better protection in order to get rid of some ads that could easily be closed out of anyway. I used MSE before switching to Avast, and it wasn't a good experience. After installing Avast, I've never gotten a single piece of malware, and I'm not about to give up good protection just to get rid of some advertisements.