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They're not the most well-known ab isolation exercise, but they are documented on exrx and Icecream Fitness 5x5 recommends them as the only ab isolation needed in the routine.
Does anyone know/have a reference to whether they are worth doing over regular crunches or other core work?
They are a pretty fun movement.
If they are fun, they are more effective because you are more likely to do them.
As far as them being more effective in actual muscle building, it all depends on form. If form is correct, then they are not any more or less effective than any other ab isolation exercise done with correct form (With the caveat that cable crunches are often easier to do with higher level weights).
The primary function of the rectus abdominus is flexion of the lower (lumbar) spine. Depending on whether you anchor the pelvis or the chest, it will move one or the other towards the opposite. (Anchor pelvis, it will move the ribcage to meet the pelvis in direction, and vice versa). Anything that increases resistance to that movement will have beneficial effects.
I wouldn't suggest do kneeling cable crunches unless you need the advanced weight to actually engage your abdominal muscles - meaning you are a very advanced lifter or body builder.
Anytime that your have that much contact with the floor to settle your body - knees and feet in this case - the more the weight is distributed. In fact I would strongly suggest that this lift is often done improperly engaging in too many muscles outside of the abs. I considered myself very advanced with ab workouts and wouldn't touch these. I don't want to over-generalize but I feel my hip abductors and shoulders getting a workout - not good.
If you really want to work your abs out - get your feet (and knees) off the floor.
You LITERALLY ONLY work your abs on Cable Crunches when you are consciously, tightening and squeezing your abs. Cable Crunches even in the correct "form" will only give a small amount of benefit to your abs. They are however, a GREAT upper back exercise- but ONLY the upper part of the lats.
Depending on how you do them you will not fully workout the lower part of your abs and mostly the upper part. I would not do them, as you say "over regular crunches", but in addition to. In order to get a complete abs workout done you will need something like this:
Warm-up
Sit-ups, max amount possible
Workout
Hanging Leg Raises
Hanging Leg Kicks
Hanging Side Leg Lifts
Hanging V Lift
Wood Chop, High to low
Wood Chop, Low to high
Twisting Sit-up, Incline
Leg Cross Twist Crunch
Doing that once per week to begin with and you will have a complete abs workout.
Bottom line however, if you think the exercise you are doing is fun and your seeing results there is little reason to change.
