I think it makes all the difference...
If you took ten non-interventionist winemakers, with at least some experience, gave them the same grapes -- all destemmed, gave them the same barrels, told them how long to age the wine in the barrels, and gave parameters for lees stirring, etc, I suppose it wouldn't make a gigantic difference.
However, this is never the case.
While vinyard site, vintage, and vinyard manager are all crucial components to a wine, remember it is the winemaker who performs the magic of turning grape juice into wine. Without going into a lengthy process which I do not begin to pretend to fully understand, let me simply point to some winemakers and their impacts on wine:
Michel Rolland has managed to produce Clos Apalta (WS #3 WOTY, probably the best Chilean wine) Yacochuya (one of the best Argintinian wines) consults for CA Cabs/blends such as Harlan, and produces many of the best St. Emillons and Pomerols
Ricardo Cotarella is responsible for the *majority* of Central and Southern Italian wine I have enjoyed over the past few years. (La Carraia, Di Majo Norante, Morgante, Falesco, Monti, etc all use him)
I simply cannot come up with a reason why some of Helen Turley's wines taste as good as they do; but, Pahlmeyer has not been the same since she left.
There are many great winemakers out there and these three were just ones which I could make an obvious point with. The truth is that I do not believe that there are any winemakers so good that they cannot be replaced (for example, Landmark has continued to thrive since Helen Turley's departure). However, just because there are many great winemakers, does not mean there aren't a whole lot more bad ones.
(I'm sorry if this post seems argumentative, I'm really tired and I did not mean it to be so, if it is.)
"What contemptible scoundrel stole the cork from my lunch?" -- W.C. Fields