I haven't looked into ADX yet so I can't give a full list of reasons, but Google were just fined $2.7 billion by the European Commission for some pretty shady business with their Shopping Service. Hopefully decentralising the network will avoid instances like this!
Also, just like the rest of crypto it would bring much needed privacy to our lives. If your a user of gmail, Google sweep your emails and let companies target you based on keywords you've used in your emails. This isn't the only thing they do along these lines either! I think there is numerous good reasons why a decentralised ad network would appeal to people.
Sure, I can see why it would appeal to people who value their privacy. However, they are not the customers on an Ad-Exchange. Marketers are and publishers are.
There's loads of advertention networks online. What you see is that the ones that offer the most data are able to ask more per click or per 1000 impressions. Publishers like earning money, so of course they choose the network that pays the most. Take Facebook for example. The ammount of targeting you can do on your ads is ridiculous. And that is what appeals to marketers.
So if privacy is the selling point of a decentralized Ad-Exchange, I'm not conviced it will be a success. And even if it would turn out to be a great selling point, what's stopping normal centralized Ad Networks from providing privacy?
Now there could be an obvious advantage that is staring me in the face, so if anyone could point me towards other advantages of a decentralized Ad-Exchange I would be most grateful.