I think I heard mention in one of Direwolf's vids that Slowpoke thought Soul Shards was too unbalancing, and since he's the head of FTB, that might very well be why... I hope he is persuaded to include it. It does take significant effort and the only quick way to do it is to find and kill a spawner that you could have just moved with the Portal Gun anyways... In the case of Blazes, you even have to destroy three (or was it four?) of their spawners!
The end result is only somewhat more convenient than a normal mob grinder, and I suspect people will use the normal mob grinders anyways to help speed up the process of charging up a Soul Shard.