Allowing officials to take brown envelopes is a terrible idea. Everything about Thailand's administration (and that of many other countries) is a terrible idea and topping a few to "encourager les autres" would get my vote (er, I don't have one).
I don't say all officials are bad, but all are bent. Even the good ones - who do their jobs diligently in their community - will happily accommodate one in return for a little something. And that is about something else - the rate for the job.
To be serious, "Pay peanuts, you get only monkeys". The diligent local cop, or local council guy, gets a pittance. No great incentive to refuse my occasional "pourboire", is there? But - and we all know the reality - pay better and you don't actually get rid of the corrupt(ion). You have to somehow weed out the baddies BEFORE rewarding the rest. And that is something no administration in the region, perhaps in the world, has learned to do.