Pick a bad metric to measure and you’ll adopt poor methods. For example, if high average time on site or page views/visitor is the metric you choose for gauging the success of your site you may end up with a bunch of visitors who are annoyed with how long it takes them to find anything.
Or if having a huge amount of visits is your metric of choice you may be tempted to use spammy techniques to get traffic; is it better to get a click and then annoy someone, or better to only reach the people who care?
I think a good question to ask is, therefore what? “I want to double the amount of email subscribers to my site,” therefore what? “So that I can get more sales.” Howabout seeing if you can improve your conversion rate from 1% to 2% on those currently in the subscriber list, which also doubles the amount of sales your email subscribers generate, and in the process creates loyal customers that will have a desire to generate word of mouth for you and improve their lifetime value?