I think this is a discussion we need to have (and hopefully Reece Richard Robbins et. al. will take to GW) because there needs to be an OFFICIAL rule on this.
With the advent of the new codexes, there are a plethora of units that aren't in there (rifleman dreads come to mind) but in what I can only assume is an appeasement move, GW has said if you have a model for it, you can use the rules in your Index if rules for it aren't in your Codex.
Sounds good, right? More options and whatnot.
...right up until the point when you realize that by not saying "Codex invalidates Index" (as it was, and as it should be) they're leaving it open to all kinds of wankery like people taking CSM Cult units as Troops (because it's in the Index). The argument goes something like this:
"I play World Eaters, and Berzerkers are Troops in Index: Chaos. Each datasheet is it's own discrete entity, even if they have the same name. The Troops battlefield role datasheet isn't in C:CSM, therefore I can use the Index: Chaos version where they have the Troops battlefield role."
What I cannot find anywhere is a definitive statement from GW saying you can't do that. The Community article about upcoming Codexes seems to say you can (though I know it wasn't what they intended). One needs to be made from an official source.
Now, as a TO I know what ruling I'll make (and it would probably involve derisive laughter followed by looking them straight in the eyes and asking, "Are you serious?") and I'll bet 99% of the TOs here agree with me. But this is something that could have a real negative impact on the community and the overall health of the game. GW needs to close this loophole and stop the rules lawyers in their tracks before this crap spreads.