I always play SP games with 75% magic sites, I love a magical rich world. I still never have enough gems...
Also I do not empower mages usually unless I am desperate. Nature gem shortage is always a particular problem.
Having said that I have'nt really noticed much of difference. I have a tendancy to be lazy in SP and cast Arcashic Record to find my gems.
I can't see it giving too much a advantage to blood, it takes alot of micro to get 100 - 300 blood slaves and ties down alot of mages.
I think its a very minor problem overall.