It's the age-old dilemma between marketing and user experience. We want to push our messages out to as many relevant people as possible via effective, available and innovative channels but not at the expense of the user experience. It's a fine balance between hitting your conversion metrics, trying to offer your visitors relevant and timely information whilst not disrupting their journey. 

This is a great summary of what to consider when launching a new overlay so that you can get that balance right.