Interesting one this, as if I had my way EVERY referee and observer would have to pass a LoTG test at least every two years. There are too many referees active out there who are applying laws from 5 years, 10 years, or even longer. All referees get notified of new laws, but do all of them actually read them, and if they do read them do they necessarily understand them? I've seen a referee this season insist that the kick off must go forwards, no idea what rock they have been living under, and it doesn't really affect the game, but that is embarrassing. I've also seen referees apply archaic offside laws.
In other professions you have to re-certify regularly, and I think I am right in saying that rugby referees have to do this. And with technology these days it could easily be done on line - for example answer 20 questions in 30 minutes which gives the referee little or no time to refer to the LoTG as they take it.
Never going to happen I know. As lincs22 says though, at L4 and above it will, and it is vital that referees know the laws inside out, but probably more importantly the competition laws. There are so many stories of games having to be replayed because referees have incorrectly applied competition rules, normally in relation to whether to play extra time or not. Make that mistake and your reputation and career is unlikely to ever recover.