A lot of what has been said here isn't quite accurate. Codex's aren't meant to be a limiting factor in an army, they are just a set of rules for playing the game. Rather than reproduce the same rules over 4 'dexs, they have separated them into clearly identifiable factions (not defending the decision,
btw).
If you want sternguard/stormravens/centurions in your dark angels army, buy them, paint them dark green, and use the allies/unbound rules to make it playable. It might be a clumsy approach for some, but that was how it was intended to work (and help sell more sm codices).