One of the things that having books come out more quickly means is that the armies are better balanced. Not because the writers do a better job of making sure their work grooves with the rest of their peers, but because you have less problems thanks to time creep. With the exception of SoB, you no longer have properly "old" codices anymore that were written under a different paradigm, for a different kind of game.
It won't be long now before ever codex has a 6th edition version of it that has had at least some amount of work done on it to make it a 6th ed codex. This means that you should no longer necessarily choose which army you play simply based on which one is newest.
Which frees things up a little bit more to make a choice based on other things.
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