Generic companies have been successful in helping FDA maintain key products the market needs. These niche products are aimed at a small patient population. The fact that there is a small patient population does not mean certain conditions are less severe. Lou Gehrig's disease and premature babies require a host of drug products, and generics are maintaining the supply needed by those populations.
Ultimately, FDA wants consumers to feel confident. Brand or generic, the consumer is getting a FDA-approved product that is interchangeable.
A good source of drug information is the Orange Book. It lists all approved products, brand or generic, except monographs. Generic companies use the Orange Book to see which products to do studies against, and which patents and exclusivity are in effect. Brand or generic, companies can see whether drugs are interchangeable. Even if there are multiple brands for the same disease state, FDA will determine whether or not they are interchangeable. Access the Orange Book at http://www.accessdata.fda.gov/scripts/cder/ob/default.cfm