I will probably get down-voted for that, but personally I do think that the AfD adds value to the democratic system in Germany. In my whole life I haven't experienced a single other topic that's so controversial and influential as the whole immigration/refugee-crisis. Merkel gets criticized a lot for her open-border policy, but actually in Germany her party is still the most-restrictive in this regard except for the AfD. You can see that e.g. in the current discussion where social democrats want to allow all refugees to bring their relatives to Germany (right now this is only possible for a part of them). Merkel first wants to see if the number of people is manageable for Germany and want's to make the decision later (so after the election ;).

To show its importance even in 2017: In the yesterday's tv confrontation between Merkel and the social-democrats front runner, immigration was the first and longest discussed topic. I believe that even in a country with Germany's history, there has to be a party where people who disagree or are concerned with the recent decisions about immigration should be represented. You would do democracy a disfavor if you try to cut out the AfD or their voters from the public discourse.

I know they have some politicians with quite extreme views which they haven't got rid of. And they will pay the price for that by getting a lot less votes. Maybe it's also since I am not a German and therefore don't follow german domestic policy as close as many Germans do.

Personally I try to imagine the two leading parties in Austria having the same stance on immigration as in Germany. This would end in a disaster for them in the next election, which is shortly after the german election.