Listening to outsiders is important insofar as it helps us consider our blindspots. However, we must keep in mind they shouldn't inform our theology to change it negatively.
Books like Dave Kinnaman's
Unchristian are important to see how we're perceived by outsiders.
Knowing that many 20somethings look at our churches and see something that might not be there. Having the info that let's me anticipate their perception allows me to shape how I approach dialogue and engagement with them.
It doesn't change my theology on any of these issues, but can (should) tenderize my approach to be ever more gracious.
For instance, one of the challenges is knowing that many outsiders to Christianity think we're homophobes. Well we're not, and my position on homosexuality isn't going to change, but I can see how we're perceived to be and that allows me to make a different move when talking about or engaging with someone in dialogue about this issue.
Also, when it comes to our worship gatherings, I regularly go and find one or two families who aren't believers and invite them to come with a "secret shopper" mentality in evaluating our ministries.
Outside perspective helps.