I am currently defending a legal malpractice case for a friend of mine and have insisted that the plaintiff prove that amount of damages that would have been collectible in the underlying tort case. I have a hearing on this issue coming up shortly; the trial is this Winter.
I think the burden of proving collectibility should be on the plaintiff because it should be deemed part of the causation argument. More specifically, the plaintiff has to prove damages by reason of the alleged malpractice of the lawyer. (The lawyer failed to have process re-issued in a timely fashion, and the case was dismissed with prejudice). That means plaintiff must prove that what damages, if any, he would have been able to collect in the underlying tort action against the original defendant. The plaintiff should not be able to collect more damages from the lawyer defendant that he would have been able to collect against the original defendant. What the plaintiff lost was the right to proceed to trial against the original defendant, and therefore what he should be able to collect from the lawyer is what he could have collected from the original defendant.
Illinois Legal Malpractice Blog has a post that highlights a recent case taking the opposite view but goes on to cite cases that set forth the majority (and better reasoned) view.
There is one, unreported opinion from a Tennessee appellate court that discusses the issue and, in dicta, it says the collectibility is a non-issue. In other words, if this case is given any weight, a plaintiff in a legal malpractice case can collect more against the lawyer defendant than he or she ever could have collected against the original tortfeasor. That is simply ridiculous.