Slave culture had memories of previous attempts at slave revolts and what happened. Where such revolts succeeded, like Haiti, or at a minimum caused major issues for slaveholders, you had slave populations that significantly outnumbered the slave owners so that the military advantages were literally overwhelmed. Not the case in the south prewar, and certainly not so during the war. Yes, it might cause some diversion of military effort away from the front which might, or might not, make a significant difference. No matter what it would result in a blood bath for any blacks in the area of the revolt, and other negative results elsewhere. Slavery was bad, it could be made worse - segregation of male slaves in barracks and chained up at night only one example. Additionally, it would be inevitable that the slave revolt would result in actions against whites including murder of men, women, and children, rapes, burning of houses etc that while completely understandable in reaction to the sufferings of the slaves, would hurt the cause of blacks in the postwar period - seen as "irredeemable savages" and potentially lead to forced emigration.
If such a rebellion was officially sponsored by the Union, the Confederates might try many more Union officers under the charge of inciting a servile revolt or simply shoot more soldiers as was the case with white officers in black units.
IMHO overall a slave revolt would be a bad thing both short and long term.