Personally I think this is both the worst, and the best possible decision Ansom could make under the circumstances. Thanks to the bonus it gives against Uncroaked, it's -possible- for him to open a breach for long enough to get the rest of his troops through. At the same time he marshalls the waning morale of his allies and remaining troops, something that will fall apart completely if this siege lasts much longer. Also, he realizes on some level that the true purpose of Parson's gambit is to simply buy time; which if he gets the breach now, Ansom ruins completely.

So in the final analysis, Ansom is simply maintaining the style of leadership he's shown before, a willingness to make unwise gambles for huge possible gains when he is given no "good" options. Crazy? Oh yes, but he isn't stupid.