I agree, the debate is a good one, and you and @zantetsu are not the first to raise this exact point.
The mimicking in TowerBFT you describe is very plausible because you do gain something from not behaving correctly. If anything, this totally proves my point! 
Maybe I underestimate what length validators will go to change the code for absolutely no gain, but rest assured: if anyone ever does it, we will suggest and implement a punishing mechanism right away. Until then I trust 75 years of game theory.
BTW: It is not really easy to come up with a rule/punishment that cannot be abused. In particular we cannot say that we only reward those voting with the majority because with that it absolutely makes sense to delay your vote, and then we are back with patches on top of patches.