Why don't standard bike chains have inner/outer combo links so odd chain lengths are possible?
Bike chains have to have some other method to take up the error (like horizontal dropouts or tensioners even on a single speed). 

They'd still need this if the maximum error was halved, so you wouldn't really gain anything. Chains would need thicker walls as the  walls would no longer be in pure tension (the force would tend to straighten them). This would make the chain heavier and wider, neither of which is desirable. It would also be more expensive as parts would have to be stamped then bent rather than just stamped.