I guess it depends on how you got to the point of using your Mix as a strike ball. If you had no other options and it was all that was left in your bag, then I can see using it.
But if your intent is to have a urethane ball be used as a spare ball AND a strike ball, then I would normally not recommend the Mix. Something with a more dynamic of a core and perhaps a stronger cover is usually the better option IF the purchase is intended to kill two birds with one stone.
But if the ball is intended to just be a spare ball, then the Mix is a good option.