I think in most instances, the juices will ferment. I like to futz and tinker and measure becasue the geek in me is just dying tobreak out.
I always stir during my primary fermentations. I do this to circulate the sugar in the must and yeast so they come in contact with each other. If one does not stir, I think the heavier juice at the bottom does not get its fair share of attention. I have tested and have used a wine theif with must drawn off the top and off the bottom and have come up with differrnt specific gravities in the same 6 gallons of must. Sure, the difference was .01 but that was enough for me to confirm my suspicions that there are layers on juice in the musts. Futhermore, I feared that the alcohol would rise to the top concentrated and the yeast would begin dying off due to toxicity before the sugar at the bottom of the carboy came to the party.
Another difference maybe unrelated but neat to consider is the primary container itself. I did 2 5 gallons Cabernet juice buckets in 07, same exact lot. A 6.5 gallon juice bucket does ferment slower that the 7.9 gallon typical beer fermenter when the same amount of must is used and the msut is not stirred. I stirred each bucket well before putting into a primary, added sprinkled yeast and left them alone. I did 4 other buckets intheis same manner and the juice in the 7.9 finished up faster consistently. I sought out the differences and started taking hydrometer reading from different levels.
No airlocks for me during primary, just a towel so the yeast gets oxygen.
So that's why I stir. Hope this helps.