There are "a number of very good, legitimate commercial purposes" for allegedly dubious trading by Westpac rates traders on a string of dates around six years ago, the bank has told the Federal Court.Westpac's counsel, Matthew Darke, centred much of his closing submissions - after a lengthy trial - on the credibility of the bank's witnesses, all senior figures in the group treasury function at the bank."Your honour has the trader, Colin Roden, on oath denying that he traded to manipulate BBSW, denying that that was a purpose for his trading," Darke told Justice Jonathan Beach."Your Honour would, in our submission, have to disbelieve Mr Roden in order to find that his dominant purpose for trading on the day was to manipulate BBSW. "If he traded to manipulate the rate, he's very unlikely to have forgotten that fact," Darke said."We say, looked at fairly, one sees skilful trading by Mr Roden, buying at the best yields he can get, starting at the high point and finding liquidity where he could, getting everything he could at the best yield he could and then going down to the next level when he couldn't get more."What we say is that that transcript is entirely orthodox trading that is consistent with the purposes identified by Mr Roden and the other witnesses. "He's buying much needed prime bank bills at the lowest prices available and at prices or yields that were undeniable good value … He certainly wasn't marching the rate down," Darke said. "I ought to give Mr Roden and Westpac the benefit of the doubt in terms of the equivocal interpretation that you say might exist in terms of [a colourful] call between Roden and [Sophie] Johnston on 6 April," the judge mused."Another way of putting it is that, if your Honour were to find that, ASIC would not have satisfied the burden of proof to the standard required by section 140 of the Evidence Act," Darke said.ASIC, the Westpac barrister argued, "hasn't called a single witness to say that outright interest rate risk and rate set risk are the same. No witness in the case accepted that they were and plainly they're not. "There has been no challenge to [bank treasurer, Curt] Zuber or Mr Roden that Westpac hedged rate set risk by buying and selling prime bank bills. For that reason, it's not open to ASIC to run a case that Westpac didn't do so."