zekeigo - I think logic alone will suffice? I'm sure it is written somewhere, however.
Let's say the ILS minimum is 200'AAL, while the circling minimum is 800'. With a cloudbase of 400' you could certainly fly the ILS and become visual, but you would then have to climb back into cloud to 'circle'! Not a lot of point. Remember the minima for the ILS depend on your being in the approach and g/a area for that ILS (ie lined up with that runway), whereas the circling minima allow you to be within about 2 miles of the field, subject to any circling sector limitations.