He offered three reasons (near verbatim quotes):
1) would-be offenders would be able to game the decision making process;
2) public clamour for even more refinements would be entirely unhelpful for prosecutors;
3) would-be criminals would be less concerned with the law
For the layman, these sound like cop-outs for not being able to argue a legal case competently.
1) If a would-be offender has a "sufficient body of reasons", does that not imply he or she may not be guilty as assumed by the system? All reasons need to be considered if the decision making is to be above reproach. Whatever happened to "seen to be fair"?
2) Poor things, prosecutors will have to work harder if the public is not happy with the outcome. Reminds one of what Yeo Cheow Tong once whispered to then prime minister Goh Chok Tong, but unfortunately caught on an open mike, "If more opposition members get into parliament, our job will be more difficult".
3) Anyone, not just criminals, are concerned with the law. It is the law that determines whether one is a criminal or not. It is not the person, lawyer or prosecutor, that is judge, juror and hangman.
Apparently the question about transparent guidelines stems from the case of Woffles Wu, who was charged under the Road Traffic Act instead of the Penal Code over making his employee take the rap for speeding offences. K Shanmugam had hinted then the reason for the judicial decision would be forthcoming, but it never did materialise. This is not to be confused with the case of the puppy Woffles, which got it's owners into a legal mess for going on holiday without permission of the authorities. The puppy was never brought in to help with the investigations. Dogs appear have a better deal than humans. Just listen to the AG: "It is their right, but I don't think I should assist them in discharging their burden".