Amber extracts are made with any number of different ingredients. Briess's, for example, is mashed with Pale, 60L Caramel, and Munich malts. Thus it is quite well-suited for styles like Stout and especially Brown Ale.
I'm going to disagree with Yooper and recommend you simply steep a few ounces of cracked Chocolate Malt instead of the extra Crystal/Caramel malt. I've done a simple Brown Ale many times which consists of 6.6 lbs Amber LME and 4 ounces of Chocolate Malt, hopped with Fuggles.
The reason you see very few recipes featuring Amber or Dark extracts is because you have more control over recipes featuring Pale/Light extracts and specialty grains. The darker extracts already have specialty grains added to them to make them darker and give them distinctive flavors; since no maltster provides analyses which list the exact proportions of each, you can't know exactly what's in there, so you lose a certain amount of control. Moreover, not all extracts are the same - Briess Amber is different from Muntons Amber, etc.
You get the picture. Good luck!