Soul shards/Soul cages in conjunction with MFR mob farm machines synergize really well. One requires energy to produce mobs, but those mobs can be kill to increase soul shard tier. Soul shards provide mob essence, necessary for producing mobs via MFR mob farm machines.
I will be using soul shards on easy to get spawners, such as zombies, skeletons, cave spiders & blazes. Those should provide ample mob essence to begin spawning things I want, such as endermen, witches and ghasts. Though endermen shards are relatively easy to level up, given you have defeated the ender dragon, I do not want to spent the time to kill 500+ endermen for a tier 4+ shard.
Mob essence can also be used in an auto-enchanter to produce enchanted books, good for those who use nano/gravisuit/electric tools. I am not sure if you can enchant MPS via enchant books, but that may be another bonus.