I am approaching my yearly tour of MR 4.2, which I do not like. Other than prosecutors, however, I don't know whether others share my skepticism of the rule or, relatedly, whether I am missing some of its virtues. So here's how I see the CBA--all additions and corrections welcome.
Costs
The rule makes it harder to gather evidence that could make fact-finding more accurate. This presents costs in the case at hand and some general system-wide cost in diminished accuracy (and thus deterrent effects) of factual findings. This is a particular problem with entities and entity constituents.
The rule vests rights in lawyers, who might be disloyal to clients, if only to help out other clients. This would be a particular risk where counsel represented an entity and an entity constituent and tried to keep the constituent from cutting a personal deal, as happened in U.S. v. Taleo, 222 F.3d 1133 (9th Cir. 2000), or misrepresented the content of some communication in order to steer a client to a course of action the client would not favor if the client had direct communications. Lawyer agency costs count as costs to clients, and the rule would tend to exacerbate the agency cost problem in the (probably rare) case where it was a problem.
Benefits
Some lawyers might lie to clients directly but not to lawyers, and Rule 4.2 keeps the clients from being lied to.
Some lawyers might trick clients into doing things, such as signing releases, they would not do if they had a lawyer filter the communication.
Some lawyers might trick clients into saying inculpatory things (or producing documents, etc.) they would not say if a lawyer served as a cutout for the communication. Note that this is a private cost to the client who is tricked but might be a net social benefit if it aided in fact-finding.
By ensuring that represented parties will never have to hassle with counsel for the other side, the Rule gives them peace of mind.
For the same reason, the rule increases the value, and presumably the price (income) of lawyers for parties who might be contacted though (more speculatively) it might lower the value (price) of lawyers who otherwise could do the contacting.
Netting
The main costs can be dealt with through prohibitions on deceit, which already exist elsewhere in the rules, and in ordinary tort law. The only justification for adding 4.2 to the list is to hedge the risk that deceit could not be proved, by making the fact of contact unlawful.
But that makes little sense. There is no reason deceit should be especially hard to prove (client testimony is admissible, after all), and one could produce a better-tailored result by allocating to the lawyer the burden of proving the truth of any communication once the client introduces evidence of deceit. (As to signing releases or the like, deceit would be a defense to contract formation, I would think, and thus to the release.)
In addition, part of the costs are simply private costs that might count as social benefits; at worst they should be netted and presumed to cancel out in the wash; for ever loser from the tricked inculpatory statement there will be a winner.
Even if one does not buy these arguments, it would be hard to justify a deliberately over-inclusive rule unless one believed a large fraction of 4.2 violations involved some sort of actionable trickery or lying. (I here set aside undercover operations, and the like, which amount to lawful trickery.) I have not done an exhaustive study of the cases to see how often this is the case, but I see no particular reason to presume that most lawyers would lie if allowed to speak at all. And if lawyers are that bad, it is plausible that the agency cost problem, which the rule might exacerbate, is equally bad, so that the netting is indeterminate even on assumptions relatively favorable to the rule.
With regard to entity cases, as a practical matter all the rule does is set up a race where entity counsel tries to represent (or line up lawyers for) former employees, who fall outside the rule, while plaintiff's counsel scramble to get to them. Relatively formal arguments about the scope of representation can support the distinction between present and former employees (who might have tremendously inculpatory evidence), but the case with respect to practical arguments is much closer
Finally, there is the mis-alignment of the right. 4.2 gives the right against contact to the lawyer, not the client. This strikes me as hard to justify. The same client who can waive the right to counsel, the right against self-incrimination, and the right to put the government to its proof--who can, indeed, plead guilty and refuse to contest a capital sentence--cannot make this decision. That the rule originates in (according to Leubsdorf) 19th-Century lawyerly etiquette explains, but does not justify, this allocation.
DM