1. The dryer is expelling air outside whether gas or electric. If in a smaller space, it can lower the pressure in the area (a bit of a vacuum) and cause air to come in from other places - like your furnace, stove, hot water heater, etc. exhaust vents That runs the risk of carbon monoxide build up in the home, from those other appliances. Problem becomes much more prone in a smaller home - like a mobile home.
2. Most places have changed codes to not allow the pleated dryer vents, although they can't prevent them from being sold. Too much build of of lint, which can cause a fire.