I consider an ad-blocker a security patch.
near 100% of malware infections I've had to deal with on clients machines in recent times have come from, you've guessed it, ads exploiting browser bugs, or simply tricking the user into installing junk which then further opens up their system.
First thing I do is give them an ad-blocker (and recommend something like no-script if they can manage it)
Never had another problem, it's a better anti-vrius solution than an actual anti-virus.
Ad-blockers are the FIRST line of security, and the most important.