Tenant Taking Advantage of Early S21 Notice

Possession - Section 8 and Section 21

Hi,
I have always followed your advice in issuing a S21 early in the tenancy so that we 'Have it in place' should we need to apply it. I have a 'what if' for you to consider as follows:

Assuming I have correctly issued and date a S21 notice in month 1 or 2 (After protecting the deposit)into a 6 month tenancy agreement, what is to stop a savvy tenant leaving at any time after month 6 (Say month 7, 8 or whatever) without giving me the normal appropriate notice, their argument being they are only complying with the S21?!

Would they be acting legally and within their rights?