The key to understanding the correct outcome in Lafler v. Cooper rests, in my view, on understanding the difference between an outcome secured by a windfall versus one secured fair-and-square.
I accept that a defendant has the right to effective assistance of counsel at the plea bargaining stage. Bargaining is now the principal means through which charges are resolved, and thus ought fairly to be considered a stage of the prosecution covered by the text of the Sixth Amendment. The question is: Who pays the price if the defendant does not get effective assistance at that stage, and the matter comes up only after the public has paid the bill for a trial whose conduct and outcome are perfectly legal?
Trials are the method designated by the text of the Constitution for the resolution of criminal charges. Once the defendant gets such a resolution, he has received the principal item guaranteed him. It's difficult to understand why a person who gets a legal sentence after a fair trial has a claim enforceable against the government. In any other context, it's unheard of to punish one party for the fact that the opposing party got bad advice from his own lawyer about how to negotiate.
That is what differentiates this dispute from what happens when a defendant gets ineffective assistance at trial: When there's a realistic prospect that the government got a victory in the trial setting it otherwise would not have achieved but for defense counsel's incompetence, then it stands in the position of a windfall beneficiary, and it is not unfair (or at least not grossly unfair) to make the government surrender what may be seen as having fallen into its lap.
But that is not this case. Once the government gets its trial victory and sentence fair and square, by working for it and following the rules, to then effectively nullify that outcome because the defense lawyer erred in advising his client -- in conversations from which the government was excluded and might never even have known about -- seems extremely unfair to the public. Indeed it seems downright bizarre, and has the feel (and for sure the massive potential, should the defendant win this case) of self-dealing.
There is no good remedy here, only a least bad one. In my view, since the pre-trial defense lawyer was the problem, he's the one who should pay the price. He should be disbarred. This will not help the immediate defendant, but it will provide such a stiff dose of deterrence that, over the long haul, the quality of defense counsel will improve, and fewer such bargaining snafus will be committed.
This is an instance where one party or the other is going to have to live with some unfairness, as Kent pointed out. For the reasons I have outlined, in the trial setting, the balance of interests, all things considered, tips against the government. Where the defect is in private defendant/defense counsel strategizing conversations, however, the balance tips the other way.