A site I was consulting was getting a depressingly low number of email subscribers, so we tested one of these modal popups that prompted the visitor to subscribe... It worked wonderfully. To minimize the annoyance we set it to pop up only once for each visitor (based on cookies).
It wouldn't be so popular if it wasn't effective.
Maybe email signups were depressingly low because nobody actually gave a toss about their marketing emails, and all you did was trick a small percentage of viewers into signing up for a mailing list they have no interest in because they thought they had to sign up to close the modal. I completely fail to see how your client got any value out of this at all.
Which, if I'm not an intimate viewer of your site, means every time.
Of course, if you're visiting a site for a person or organisation that you are actually engaging with and they still do this, you are entitled to feel karmic smugness the next time they forget their umbrella and it rains. :-)
But if you're some site I'm just visiting for the first time, then no, you don't get free use of my disk and machine to track me. When I mentioned "every time", for some sites, this is often "every page load" — so I'm not a repeat visitor.