Just stumbled again upon this quote:
“Our studies showed that ads that pop up in new browser windows hurt a site’s credibility. This makes sense; people usually go to Web sites with goals in mind, and they expect the site to help them accomplish those goals. Pop-up ads are a distraction and a clear sign the site is not designed to help users as much as possible. Pop-up ads make people feel used, perhaps even betrayed.”
(Brian J. Fogg. 2003 Persuasive technology: using computers to change what we think and do)
Even though the quote mentions specifically “ads”, this is also true for newsletter subscription reminders (which may be also “ads” in peoples perception) and other stuff which is constantly popping up and therefore interrupts people.
Suppose you go into a shop and just want to look around with no intention to buy something. What impression does it make if the store owner stands behind you, constantly asking if you want to come more often, maybe tomorrow, and despite the fact that you say no, she keeps coming back, asking again and again. This is what the pop up window does, literally. Will you ever step into this store again? And will you perhaps tell your friends about this experience and tell them how crazy they are and tell them not to go there?
And: Do you want to be this store owner?