Jt, All the extracts have their place in recipe formation. In reality you could omit specialty grains and use a little amber, dark, Black** or rye instead. But as the above party mentioned most people use specialty grains to color and flavor as you do get a slightly fresher grain flavor. For the bases you would use the Pils, light, wheat or Munich DME/ LME The shift away from the amber, dark and black** extracts was because again as said above fermentibility is lower, but the same hold true for fresh specialty grain when comparing it to base grain. Sometimes in the past and in a lot of cases you should not follow a stores idea of a extract recipe if it uses high percentages of dark malt extract recipe/s as it might be 10+ years old. I have made plenty of extract recipes years ago that were 70-90% base extract and a little amber, dark or wheat DME to emulate fresh grain as it all goes in the pot, you boil and your done. The bottom line is it gives the malt extract retailers and new brewers more options such as ease or if using grain may seem foreign, plus you have a shelf stable product whereas some smaller LHBS may not be able to stock the array or keep fresh the grain the consumers want. Now with the explosion of online ordering this concern starts to fade. But I think you see what I am drawing to here and variety is due to demand. If they only offered pils malt extract then somebody would be beating on their door to produce amber etc…
In the past people would report getting funny flavors from all extract beers, But I can assure you that if you use 100% extracts with good practice such as good boils and getting some O2 into the cooled wort, attention to sanitation, attention to pitching good counts of healthy yeast and controlling fermentation temperatures you can make beers that rival some peoples all grain recipes.
The Briess site is a good start to understanding what is actually in the different extracts and it will give you a peek into what SG you can actually expect when you start making your own recipes or other techie stuff about the product.
Here is a good brochure they make that shows what specialty grains are used in each etc…
http://www.brewingwithbriess.com/Assets ... ooklet.pdf
Then if you want more specific info on each extract they have a “MSDS” for each one also.
** They have just come up with a black malt extract to sub for black malts such as in your stout question as a black grain base extract has not been available in the past hence the need to use specialty grains in your current recipe and LHBS may or may not start to stock it soon.
Also the black malt extract also brings up a bigger picture too. You would be surprised but there are a number of Pub breweries that use only extracts in their on premise brews and this is why the other colored malts are around because they are shipped to these guys in 55 gallon barrels and large sacks!!! As mentioned above these guys are using SOUND practice but with 100% extract and they can sell beers at their tap for $3-6 a pop.