Unity uses quaternions for storing the rotation of rigid bodies, but vectors for the angular velocity. Why? I thought you'd use a quaternion instead of that vector also. I don't know much about quaternions and stuff, so please excuse me if this is a dumb question.
Because not only is a Vector3 more compact, a quaternion can only represent a rotation of up to +/- 1 turn signed.
If you're storing angular velocity as "rotations per second" your object could be spinning at more than 1 rotation per second, a quaternion doesn't work for this (even if you chose a smaller time base you will reach a similar problem)
A Vector3 angular velocity can be longer than 1 unit of length to represent faster rotations than a quaternion could.
It's also much easier to work with when calculating friction, combining/adding angular velocity, multiplying by the frame delta time, clamping rotation speed (a simple vector length check), etc