I brewed with extract for many years and made both good and not so good beers. I am all grain now, but I will offer the following for those who are not in case it might help them make better beer:
With regards to extract, it is true that mash pH has been addressed and so water chemistry is less important. However, I was struck this morning by the question of whether or not one should still be concerned about the water, including both chemistry and pH. Water chemistry is used for several reasons: one to optimize the mash; two, to provide enough calcium for a health ferment, and three, to provide "flavors" for certain styles. Not all beers need the last aspect,but many do. Further, different styles of beers are best at certain final pH values. It thus stands to reason that even with brewing with extract, you should 1). Set the pH of the liqour to 5.5, and 2) adjust ions to a) give enough calcium and b) address certain flavor profiles, such as SO4-- for hoppy beers, Cl-- for malty, etc. I wonder if this might significantly improve the quality of extract brews.
I might even brew and extract batch or two just to test this out...