In my experience, they'll charge you for overpacking on checked-in luggage. But carry-ons are strict, because of limited space in overhead compartments. Thus, they'll stand and watch as you go through your belongings and throw it in a bin until you reach 7kg's.
Not the point though....just because they sometimes don't ask for it, doesn't mean it's not required. And FYI, after numerous recent troubles Thailand has been experiencing with bummed out expats, they are gearing up and pushing immigration to double check the border-crossers.
Just have a peek at Pattaya news or Bangkok Today.
They're fed up using so much of the police force's energy cleaning up after foreigners with no substance.
Fully agree with you. That's why they have this 'rule'. To keep the beach-bummers-overstayers- working- ilegally people out. If you have it, show it. If you don't, case in point.