said by Kearnstd:
I think the lack of QC from ad vendors really helped drive this, Adblock is now seen as not just a tool to stop those annoy rich media ads that mixed at double volume just like their TV counterparts. But it is seen as a security tool because ad vendors who are the ones that actually supply them have allowed people to push malware via ads(ie "Your computer is infected, click here to clean up now") that popup ad alone has caused many hours of tech support.
After dealing with that when I worked in tech support, after helping someone clean their PC I always directed them to Firefox and Adblock Plus.
DING DING DING!!!! WE HAVE A WINNAR!!!!!
Ad servers rarely check their ads and when it does infect a system, they turn a blind eye to it. That is the reason Ad Block is so valuable.