1. World Eaters, Thousand Sons and Death Guard are composed entirely of Cult units. True, there's the whole "warband" composition of Chaos armies in fluff at the moment, but consider that the skirmish level of 40k could very easily represent the size of a small warband from those Cults. It would not make sense that the only warbands of any real size are led by what many consider "outcasts" from those Legions.
2. As they are, Cult units are not deserving of an Elites slot. They're good as Troops, but they cannot stand up to the Elites choices of most other MEq armies.
The way I see it happening is that the fluff is going to be retconned to make Cult troops to marked CSM what Chosen are to unmarked/undivided CSM. The Marks of Chaos will probably make regular units similar to how Cult units are now, while the actual Cult units will be more like the elite version of those, which would tie in with TDG's speculation that Berzerkers could get rending (or some other rule for Chainaxes), for example. It would also make Wes's suggestion that Cult Terminators could be an upgrade for Cult units make more sense.