The problem is even the unobtrusive ads are virus filled. Do any search for any major software package (Examples: VLC Media player, 7-Zip, Libreoffice) on any of the major search engines to see what I mean. I get no less then 3 virus infections per week just by people clicking on those types of ads.
It was much better when Google, Bing, and Yahoo put a big colored box around the ads so you can know for certain what your searching for. Google just has the yellow Ad gif now, and yahoo and bing just says "Ads related to" text which just blends into the real search results. I won't even talk about Ask. DuckDuckGo seems to seperate better since it lists official sites but even they just do the Ad gif thing google does.
I'll turn them back on when they clearly label what an ad is so that even an computer idiot can tell they're fake results and they actually screen ad URL's for misleading or malicious content (Not just for malicious scripts and the like. Obviously fake Installer Trojans that you download and execute are bad too), until then, unobtrusive ads stays off.