While I don't doubt the power of popups if used judiciously, most sites fail in that regard and just end up driving off visitors. Also, as of January this year, Google has rolled out a penalty for sites in its mobile search that use "intrusive popups and interstitials".

So if a website relies significantly on mobile traffic to drive sales / leads, they might want to reconsider traditional popups / interstitials in favor of something else.

Source: http://searchengineland.com/interstitialgeddon-google-warns-will-crack-intrusive-interstitials-next-january-257252