Pop-under: Really that bad?

We're currently discussing to add pop-unders to one of our platforms to get users to use one of our notification features. As you can probably imagine, as a UX person I'm really against the use of pop-unders. But I need some data or at least very strong arguments (because on one of our sister platforms, they do convert really well).

Are there any studies on pop-unders? (didn't find anything relevant so far...) Any other thoughts?

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Update: Found one study (daipratt.co.uk/popunders/), but it's about ad revenue (so not exactly my case).

Update II: To clarify, this is what we would do: After using the search on our classified website the pop-under opens. The content would be something like "You were looking for XY, if you are interested in getting new results as soon as they appear please enter you email address here"). So it's not third party advertising and might even be useful for the user. But to be honest I still hate the idea - I just don't like when my browser opens stuff I didn't ask for.

Update III: Great answers, thanks everyone! I just accepted the one with the most votes, but the other answers are very helpful as well.

Update IV: So it turns out that the conversion rate of the pop-under on our sister site was actually dwindling over the past years - probably because of built-in pop-up blockers. Good news for me: No pop-under on our page, yey! :)