Manadatory testing would benefit punters who engage in DFK, OWO, RO and Rimming. It would lower risks to them and their other sexual partners (paid or unpaid).
I do not condone BB, and think that anybody that pursues it is nuts, but testing is not just about barebackers.
The argument that the prossie could have been infected between tests is valid, but by systematiclly enforcing regular testing the probability of this is lowered. In some jurisdictions the testing is weekly not monthly.