Exactly my thoughts. If you think about it, it'd even be fluffy if they kept them elites; Cult units would be interested in spreading their god's work, not standing around holding some random communication beacon like a common troop

.
That's a bit flippant, but I don't think it will be bad for Cult legions. 6th will likely change army makeups in some way, so basing it off 5th perceptions isn't useful. Also, as IcedAnimals noted, you don't need fully Cult troops to make a fluffy Cult army; the Thousand Sons using Cultists makes a ton of sense, and I'm surprised I hadn't thought of that before. That, combined with a more robust Marks system, would mean you don't really need Cults as troops.