Check out the following article by Dr. Tim Keller, founding and senior pastor of Redeemer Presbyterian Church in Manhattan, New York. I think hosting a Q&A time is a good idea, but the minister must not be faint of heart, nor should he be unprepared. For the prepared apologetic minister, a Q&A time is an excellent way to reach the skeptical and unbeliever who attends Sunday service.
Here is the link to the article: https://www.thegospelcoalition.org/article/5-reasons-to-host-qa-after-worship-service.
(C) July 31, 2016. Brian Chilton.