The decision of Andrew Mitchell to resign was undoubtedly the right one. Having lost the respect of many of his Tory colleagues, he entirely lacked the authority necessary to perform his duties as Chief Whip. As David Davis astutely observed  two weeks ago, "What does a Chief Whip have at his fingertips to deploy normally? Well, a mixture of charm, rewards, appeals to loyalty — all of those are diluted at the moment." It would also have suited Labour for him to remain in place (despite the party's public calls for his resignation, it privately hoped he would survive), another consideration which will have influenced Mitchell's decision.
But his departure (announced on Friday evening in classic Westminster style) leaves David Cameron notably weakened. The Prime Minister could have demonstrated strength by sacking Mitchell and asserting that "there is no place in a compassionate, one nation party for for those who behave disrespectfully to the police." Instead, he prevaricated, neither sacking Mitchell nor backing him. As I noted after PMQs  on Wednesday, Cameron "couldn't summon a word in defence of his Chief Whip". Having shown similarly poor judgement in the cases of Lord Ashcroft and Andy Coulson, it is remarkable that the PM failed to learn from past experience to kill the story at birth.