User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170423170210/@comment-31137559-20170423202215

Taking Bennia away from Sona would be unfair. Not because of some battle strength being loss but because she's the only devil to get a Grim Reaper in her peerage and unless things change drastically, no other devil will get a Grim Reaper as their servant. Well Bennia found her and made Bennia her servant. I know that a Valkyrie, holy sword wielder, and a healer are also rare to have in a peerage and Rias traded them away. But Rias has a bunch of unique servants. Sona only has a few. Bennia is just too rare of a find and it would be too much of an asspull(excuse my language) on Ishibumi's part to suddenly have her become Issei's knight when she has expressed no desire before. And you guys can argue about Rossweisse but there was the fact that Rossweisse was a genius and would be of better use to Issei than Rias. So no more trades and time for Issei to find some potential servants.