Cost in adding depth, potentially taking losses in the league/cup, game shape will take a hit. Since the b3 is the toughest competition you can't hardly expect to win the final prize, even throwing all your resources into it. So much comes down to the draw.
Practically thinking, it's prob better for most teams to focus on league/cup, but giving up on the b3 will be hard thing for any manager to do.
"Well, no ones gonna top that." - http://tinyurl.com/noigttt