resonant mode selection for frequency meter?

There is a paragraph in Pozar's 'Microwave engineering(318, 2nd)': "Since frequency resolution is determined by the Q of the resonator, the TE011 mode is often used for frequency meters because its Q is much higher than the Q of the dominant circular cavity mode. This is also the reason for a loose coupling to the cavity."

My question is:
(1) Why the TE012 mode is not used for frequency meters, its Q is even higher than that of TE011.
(2) How to comprehend the last sentence "This is also the reason for a loose coupling to the cavity"?

Thank you for your interest and time.

Anyone help?