Waiting area or not, as long as there are children involved, you get to deal with pain-in-the-*** parents. Gossiping in the dojang will happen with or without a waiting area. Parents network, exchange phone numbers and email addresses... I even had one parent chasing another parent down in the parking lot to gossip.
In the last 20 years I have seen it all. I am a firm believer in having a closed off viewing room, making sure your restrooms are in the front of your dojang so that parents never have an excuse to set foot on the mats. Other than that, there is not a lot that can be done about it.
I can say this, I have kicked out more than one student because the parent cost the school money. If I have a parent referring students to purchase gear from the internet or pulling parents aside to bad mouth the school, I courteously ask them to find somewhere else to train. If they are happy with the lessons their child is receiving, they should be spending their money with us, to help us afford to provide the best possible program for their child. If they are unhappy with the classes, they should be training somewhere else anyway.
All of the finacially successful schools I know of have viewing rooms. This lets the parents feel more involved, helps develop report with the families of the school, and gives the instructor incentive to provide excellent lessons.
On a side note, viewing rooms are nice because sometimes the parents need the moral and character lessons as much as their children do. It is nice to have them listen in and hopefully pick up a thing or two.