This is something that Christopher Nolan understood with his creation of The Dark Knight trilogy.
We want to see Bruce Wayne break a sweat and bleed because that’s what makes him human — and therefore interesting.
Zack Snyder needs to balance large and small scale action, and Deathstroke affords him that opportunity.
The sequence in which Batman rescues Martha Kent during Batman V Superman: Dawn of Justice will go down as one of the single best superhero fight scenes of all time, but none of those two dozen henchmen ever had a chance against The Dark Knight.