The beauty of using a multi gang switch like a 2 gang switch or a 3 gang switch is the house owner will save considerably on material cost like wires and extra switches.
On the material side, imagine if you have a bedroom where there are 4 lights. Two wall lights and two bedside reading lights. Now if you were to place all the switches in different locations, it will involve using a lot more wires to run all the wires to the different one gang switches. However by coupling 2 switches together and using one 2 gang switch instead of 2 one gang switch, you can save up considerably on wires used.
An example, for the two wall lights, have one 2 gang switch located near the bedroom door so when ever you walk in the first thing that you can reach will be the switch. You don’t need to grope in the dark looking for the switches. For the bedside reading lights, have one 2 gang switch installed at the center so if you are sharing a bed, it will be convenient for both of you. The cost of one 2 gang switch is only partially more than a one gang switch. Buying 4 different one gang switch instead of two 2 gang switch will definitely cost you more.
Other than the savings on the materials cost, labor charges can also be drastically cut down. When the electrician will have to run wire to four different locations, all he need to do now is to install wires at only 2 different locations. It almost cuts down his work by half.
The only disadvantage that I can think of is when one of the switches becomes faulty. In this case, the whole 2 gang switch will have to be replaced, but then, a good quality switch will last you years and years.