The way it worked on windows is, I would go to a site, say a popup, or advertisement is on the page... somehow loads some junk on the computer which my Spybot Search and Destroy picks up and removes it, even in cases when there are NO popups from the site or are blocked from adaware (most likely due to serverside javascript maybe)
Say I had an ad blocker as you suggested, and decided to go to a site that may be sketchy like those big content farm sites that has nothing related to what I was searching for... and I left the page....
I would be able to emerge unscaythed? If there is even the slightest possibility that something adware or spyware could potentially be loaded on my computer... it would be nice if there was some mechanism to at least kind of know about the junk being put on my machine and how to get rid of it.