If the cursed earth gets too much light it lights on fire. At least it did I know there was talk of only having that happen when exposed to sunlight but I don't know if that version is out/in the packs yet. So keep it dark and be warned, it spreads to other dirt blocks like grass does (only it needs darkness to spread). Also: the mobs still despawn if you are 128+ blocks away last time I checked and so you either need it close or chunkloaded in another dimension.
And if it is exp/mob essence you want, there are a few other methods to getting it. The essence berry bushes from Tinker's construct can produce a large amount of xp/essence. You just need some way of harvesting the berries, an autonomous activator to use the berries and something to collect the xp orbs like a sewer (for mob essence, don't stand on it or you will clog the system) a brain in a jar (from thaumcraft, hit it and you get xp orbs back), or a vacuum hopper (for liquid xp from another mod which can be used like mob essence, but not in the MFR machines)