User blog comment:B214/Talk About What You Want! 3/@comment-72.240.208.36-20150415054601/@comment-26209278-20150415074533

Without the initial booster from the evil pieces, Issei wouldn't of had the strenth to summon the BG, so im not sure how much use he would of been. They would of had to train him just to be able to use it, which they would rather kill him and find another host.