Why are there generic ship class cards in Star Trek Attack Wing?
See that yellow starburst to the left of "U.S.S. Enterprise-D"? See also that there's no yellow starburst to the left of "Federation Starship"?



That starburst is the "Unique Icon" which denotes that the card is unique, which means you can only have one copy of it in play at any given time.

Being a Unique ship, you just can't have multiple U.S.S. Enterprise-Ds running around at once, no matter how awesome (and canonical) you might think that'd be. The Federation Starship, on the other hand, is not Unique: This means you can make a whole fleet of Galaxy-class starships if you wanted to (within your squadron point limit, of course), regardless of whether or not the U.S.S. Enterprise-D is already there.

Other than that, I guess maybe you might want to save two points and bring in the cheaper 26-cost Federation Starship instead splurging on the better 28-cost one.