Grilling vs BBQ semantics aside.
A gas grill doesn't add any flavor of it's own. Some of the food will sear, char, or even drip and catch on fire. These will produce a variety of new flavors that range from delicious, maillard goodness to sooty, bitter, death. These new flavors are not a function of the open flame but of the very high, direct heat.
In this regard, the only advantage of a gas grill over the broiler in your oven is that it is:
- more accessible to work with and
- located outside of your house so you don't have to deal with smoke or excess heat.
Maillard reactions require the food to be dry. You don't want your marinade to pool around your meat because that isn't grilling- that's braising. Delicious for ribs but not so great for chicken.
Her other argument for the foil was ease of cleaning. Just leave your grill on high for 10 minutes after you finish cooking and any food particles will disintegrate easily under your brush.
In my opinion, unless you do intend to actually braise your meat, the foil is completely unnecessary.