Antennas like this are very convenient for field operation. An ATU can improve transmit power transfer and receive sensitivity.
A: Yes. You'll get the maximum performance out of the KX2 when you're using less-than-optimal antennas. For instance, the KXAT2 was designed for portable, light weight antennas such as a wire thrown into a tree with some counterpoise wires (also known as radials) resting on the ground.
A: It will depend on whether you intend to carry the KX2 with you for portable use or not. I
The KXAT2 fits inside the KX2 case which makes it a fully integrated tuner for the KX2.
If you intend to operate in your shack at 100 watts, then you'd likely need the internal KXPA100 antenna tuner (the KXAT100) but not the KXAT2.
If you're planning to operate the KX2 both in the shack with the KXPA100 but also take it out for portable work, we recommend the KXAT2 be installed. This makes it easy to use one radio for both operating modes - the best of both worlds!