[13lackGu4rd]

main problem with it is that most SCs and some thugs already put the 5 spell slots to good use with various buff skills. usually there's just no room. other than that, there's the issue of having the slaves constantly handy(just takes more micro management as you can have a scout with 30 slaves following your SC and keep supplying it) also there's the fatigue issue as you said, which requires the another blood spell so 2 slots instead of just 1. also blood slaves are very fragile, once your opponent sees what you're doing he'll target your blood slaves before you can use them for your 2 buffs, and as an SC he'll usually be on the offensive(so enemy gets to act first) which will instead put 2 AI spells, which can be horrible and too often mess up with your fatigue that you so carefully tried to manage with smart scripting(hopefully). also keep in mind that you'll need 50 blood slaves for each SC, it gets very expensive very fast, so only real blood nations would be able to afford it, and those usually have much better things to do with their blood slaves in the first place...