Put it this way, Blackbeard represents an ultimate goal for Luffy in his journey to be Pirate King. These guys are the most worthy after the Straw Hats for that title.
If Luffy is given Hell to take it, so should his crew. It's a lot less worthwhile if our greatest roadblock's strongest subordinate can't push Zoro to his brink and make him grow. It would just be a really bad way to end this journey. Shiryuu is going to be an absolute beast, likely on par with Linlin of Fujitora himself.