I remember the days when I first started using the internet and the worse thing to come across was websites that bombarded you with popups - not just adult based sites, but security ones or personal ones made on free services such as Homestead, Geoshities, Lycos, Bravenet, etc..
However during a conversation of the good oldť days with a friend of mine a few weeks ago, we decided to be brave and do an experiment by browsing a few "naughty" sites using Internet Explorer 6 (the best version they did, before Microsoft decided to bulk out the browser with loads of other crap) and make sure there was no popup blocker installed and see how many popups we get.
Now I was hoping for the usual load of popups, along with diallers wanting to download (but then again, now that we're all mainly broadband, diallers are pretty much rendered useless). So, we started the test. We went onto Google and searched for "free sex videos" and checked the first three results that came up. The websites loaded and guess what? Not a single popup nor pop-under from any. Closed them pages and no pop-outs either. It was only the fourth site we checked that chucked up a little popup for some gambling site.
So, in conclusion, I don't really see the need for modern browsers to use popup blockers anymore as that trend has died out... so why not shave a few MB off the overall file size of your product, by removing the built-in popup blocker.