I never claimed every single one used blood magic, but Duncan notes that there are Grey Warden mages who use blood magic to deal with the darkspawn, so my point stands.
Just because they use it doesn't mean that they need to use it. There is no intrinsic advantage of using blood rather than lyrium when fighting darkspawn... it's a useful backup plan, can save money, and will make them more independent from the dwarves and Chantry, but that's pretty much where the advantages end...
You're missing my point that blood magic has useful applications.
No, I understand your point. I am saying that blood magic is not needed in these cases, in spite of the fact that it can be used as a substitute for lyrium. Blood magic is an alternative method of spellcasting, but one that carries with it a greater risk for negative side effects and consequences. Therefore I cannot see a logical reason why it should be legal. We already have lyrium. That should be more than enough unless a mage's goal is fighting Templars, controlling minds, sundering the Veil, or summoning demons...