I came across this question when doing numerics with the Python package pint
, where angles can be specified in $\rm cycles$, $\rm rad$ and $\rm deg$ (and some aliases, such as $\rm turns$, $\rm revolutions$).
... and then I ran exactly into this situation: I needed to calculate an angular acceleration from a torque. That should be, for constant moment of inertia $I$,
$$ \frac{d\omega}{dt} = M/I $$
but when you think of angles as a quantity with dimension - in my case angular velocities given in $\rm revolutions~per~minute~(rpm)$, it would be a unit mismatch.
Ultimately such things come down to conventions. If we argue that there is a natural unit of something, we'd end up not needing units at all; For instance we don't need the meter, we can just use light-seconds as the basic unit of length. One $\rm meter$ would then be roughly $3.335~\rm nanoseconds$.
And indeed similar situations exist. In physics, unit systems with 3 base units for length, time and mass are common, as opposed to the 7 base units of SI. The unit of current is eliminated by saying that two unit charges at rest at a distance of one unit length exert one unit of force on each other by the Coulomb law, which gives the charge a fractional dimension of $\rm (mass)^{1/2} (length)^{3/2} (time)^{-1}$.
So why have units at all? I'd say it comes down to something similar to "type safety" in programming. When you add a time and a length, you typically rightfully get suspicious. When you expect a velocity, but get a mass - likewise.
Now, in the equation above, should be add the angle units somewhere? Should we add $\rm rad$ to the torque? Probably not, because omitting units by deciding on a natural unit is not uniquely reversible. We don't know if we should introduce $\tilde\omega = \omega/\rm rad$, $\tilde M = M\rm rad$, $\tilde I=I/\rm rad$ or a mixture of all of them with fractional powers.
Also, at this point we have to ask ourselves: Are we looking at an angular velocity given in $\rm rad/s$, or is it $\rm cycles/s$? Both constitute perfectly natural units of angular velocity, though $\rm cycles/s$ is commonly written as $\rm Hz~(Hertz)$, similar to the distinction of $\rm Joule$ for energy and the technically equivalent $\rm Nm$ for torques.
Such problems are quite common when working with literature, that uses different unit systems (e.g. one of the various electrodynamic unit systems with 3 base units, vs SI). For instance the unit-less dielectric susceptibility $\chi$ differs by a factor of $4\pi$ across different unit system; This factor essentially comes down to whether we write the Coulomb law as $F = \frac{q_1 q_2}{4\pi r^2}$ or $F = \frac{q_1 q_2}{r^2}$.
The only special thing about angles is, that their natural units occur in geometry, without insights into laws of nature. But given how it is quite easy to mix up cycles, radians, and degrees (e.g. between the frequency quantities $\omega$ and $f$), maybe "angle" has as much a right to be a base quantity as "current".