(HBR) - Consulting is pervasive in many industries. Yet the use of consultants remains controversial. Why would firms give away key activities to hired guns? Why would these “mercenaries” perform these activities better than in-house employees? Many employees dismiss consultants as people who “borrow your watch to tell you the time” — and then charge you for the privilege.

To evaluate the impact of hiring consultants and to figure out when they might offer the most value, I turned to the wine industry, where over two-thirds of wineries hire consultants to improve the quality of their wines. As a winemaking consultant put it: “My job is to make my client’s wine better. Even if the wine or the winery is awful, we have to do our best in the conditions we have.” The underlying rationale is straightforward: Better wines can be sold at high prices. Overall, I studied 311 Bordeaux wineries over a 10-year period. Wine quality was assessed using tasting scores from Wine Spectator and Robert Parker’s Wine Advocate.

In this study I distinguished between mean quality and variance in quality. On average, I found that wines made with the help of consultants had higher quality ratings. However, they also had less extreme quality ratings. Use of consultants, therefore, correlated with middle-of-the-road, less extreme wine ratings: neither excellent nor terrible. Many outstanding wineries did not use consultants, preferring to use only in-house talent. For instance, the owners of Pétrus have never used winemaking consultants. From 1963 to 2007, wines were made by Jean-Claude Berrouet, an in-house winemaker. When he retired, in 2007, his son Olivier took over as the in-house winemaker.