Also, I think when there are multiple species with the same dual types, it sort of cheapens the combination and the Pokémon really needs some special stats and abilities to make up for it. Like Pidgeot/Fearow/Dodrio/Braviary, etc. Normal/Flying just isn't as appealing a lot of the times as Normal/Type other than Flying or Flying/Type other than Normal. There are a few exceptions, but I think Pokémon with unique combinations (Heracross, Gliscor, Sableye) are more useful.
They can get away with pure Grass and Water starters because they aren't as specific of types as Fire/Fighting. With Fire/Fighting, it's more obvious that it's being reused, since it's such a specific type combination.
http://meta289.deviantart.com/#/d4j1eby Fact: Things are so much better when taken at face value.