Except for that small class of errors which are apparent, open and obvious to a lay jury, an expert is needed for either side in a legal malpractice case.  As an example, and although the case does not describe the expert’s testimony, its apparent that defense needed this expert to win the case.  In SCG Architects v Smith, Buss & Jacobs, LLP   2012 NY Slip Op 07288  Decided on November 7, 2012  Appellate Division, Second Department  the AD decision leads off with:

"The Supreme Court providently exercised its discretion in denying the plaintiffs’ motion to preclude the defendant’s expert from testifying due to inadequate expert disclosure pursuant to CPLR 3101(d). CPLR 3101(d)(1)(i) does not mandate that a party be precluded from proffering expert testimony merely because of noncompliance, " unless there is evidence of intentional or willful failure to disclose and a showing of prejudice by the opposing party’" (Browne v Smith, 65 AD3d 996, 997, quoting Hernandez-Vega v Zwanger-Pesiri Radiology Group, 39 AD3d 710, 711; see Aversa v Taubes, 194 AD2d 580, 582). Here, the defendant’s expert disclosure, although not detailed, was adequate under the circumstances, and the plaintiffs failed to show that they were prejudiced thereby (see Flores v New York Hosp.-Cornell Med. Ctr., 294 AD2d 263, 264). "

The rest of the appeal seems to have been more or less generic:  "The jury’s finding was based on a fair interpretation of the evidence, and thus was not against the weight of the evidence (see Nicastro v Park, 113 AD2d 129, 134-135).

The plaintiffs’ remaining contentions concerning the jury charge and certain comments made by the trial court are unpreserved for appellate review, and we decline to address them in the interest of justice (see CPLR 5501[a][4]; Schlecter v Abbondadello, 5 AD3d 582, 583)."