I agree that it is not really too much overkill, especially if you are a tinker who likes to add capabilities and functions.
Here is an example build:
The empty spots are the equivalent of the switch being off.
1 Ailerons (Roll)
2 Rudder (Yaw)
3 Throttle
4 Elevator (Pitch)
5 Empty / Arm
6 Empty / Angle / Horizon
7 Empty / Airmode
8 Empty / GPS RTH / GPS hold
9 Empty / Headfree / Magnetometer (header lock)
10 Empty / OSD (Turns OSD on and off)
11 Empty / Telemetry
Lights
12 Lights off / Lights mode 1 / Lights mode 2
Landing Gear
13 Land Gear Down / Gear Up
Gimbal
14 Camera Gimbal Auto / Gimbal Manual or Gimbal Assist
15 Gimbal Yaw
16 Gimbal Pitch
17 Gimbal roll
You could live without many of those features, but this is an example and there are actually still more you could add like Barometer, Sonar, channels for calibration, blackbox, Heading adjust, and beeper which makes finding a quad easier.
I show you this not to say that you need 16 channels, but to say that you can legitimately use all 16. I personally like to have at least 8 channels, which is certainly a good number, however on some test builds, or planned future builds I'll be using all 16. If you are looking at buying a 16 channel, but are hesitant to put the money to it I'd suggest a modular radio that you could later upgrade to 16 channels like a Spectruim or Frsky Taranis. I presonally use the Taranis, but the Spektruim is also a good radio. If you already have a 16 channel radio that you used in a different hobby then I'd suggest just using what you already have as it is certainly better than a radio with fewer channels and will do you just as well if not better.