When just implementing a first person type control/viewport I found it way easier to just use euler angles and the gimbal lock didn't really feel unnatural to me. So I think there is something to the idea that maybe you don't even need the complexity of quaternions at all dependent on the usecase.
First person views typically only use two axes of rotation, so of course gimbal lock will not be a problem. You are not dealing with general 3D rotations.
However, as soon as you want a 3D rotation (eg. in a flight sim) then you really need a way to represent them that doesn't suffer from gimbal lock or have any singularities.
Note: even with an FPS, you do typically want some more complex camera movement (for certain actions or animations that tilt the camera) or need to orient something relative to the camera (like the gun). Euler angles do not allow you to compose rotations, and so for this you'll need to go via an intermediate representation such as quaternions or matrices. Probably this is handled by whatever engine you are using though.
Any rotation around the unaligned axis will free you. The problem with gimbal lock for 1st person controls is mostly avoided because roll is limited and you don't need to smoothly rotate over the poles anyways.
But, I find that I often need smooth, arbitrary rotations for other things, so I just use quaternions everywhere. When using Euler angles as input, I immediately convert them to quaternions.
