From Wiki: In 1989, at the height of his NFL career, the Cowboys traded Walker to the Minnesota Vikings for a total of five players (linebacker Jesse Solomon, defensive back Issiac Holt, running back Darrin Nelson, linebacker David Howard, defensive end Alex Stewart) and six future draft picks. The five players were tied to potential draft picks Minnesota would give Dallas if a player was cut (which led to Emmitt Smith, Russell Maryland, Kevin Smith, and Darren Woodson). This was claimed to be a turning point in the rise of the Cowboys to the NFL's top echelon.
So he was great for Dallas then got them a ton of draft picks. Good management.