unless you've read the completed codex I wouldn't be bashing any units as useless yet simply because you can't find a use for them in a vacuum. If rumours are correct & there are a significant combinatino of marks and icons with different abilities available as options then who knows? Cultists may end up alright. They may be alright without the icons.
4 point scoring units that can be taken in blobs of 30? sounds alright to me! If aspiring champions / HQ's go the way of all the recent codecies where you buy a unit & split them off, you may have 30 cultists c/w plasma sitting on your backfield objective with an aspiring champion making them Ld 9 / 10. 120pts + special weapons to hold an objective, difficult to remove with shooting due to the shear number of bodies & the generally good leadership of chaos leaders.
Another option is to (a) bubble-wrap your units & vehicles as you march across the open.
take a unit (whatever size) to declare a charge with your meaty assault units so that your opponent is forced to either waste overwatch shooting the cultists or miss the chance to overwatch altogether if they try to hold for your power armored troops and your cultists make the charge.
Rumours also state that DP may allow vehicles to "eat" your troops to regenerate - would you want to lose a marine to throw a hullpoint on a rhino? I wouldn't, but a 4pt cultist? sure why not?
From a fluff point of view, ALL chaos worshipers use either cultists, or press-ganged cannon fodder. Slaanesh, Khorne, Nurgle, Undivided all have a significant history of rampant cult worship; obviously the word bearers, and alpha legion are known for developing infiltrator cults; Iron Warriors make use of pressganged imperial citizens / solders to weaken the defences of their opposition before the "real" assaults take place, same with black legion; the only legions whose use of cultists seems to be limited are the night lords & the thousand sons...
Cultists belong. Sure it is possible (and probably more competitive) to take imperial guardsmen as allies, but we don't know the functionality of units in the chaos book yet. Sacrifices, bubble-wrap, Icons and cheap as shit objective holders all have the potential to make cultists solid options.