The become the biggest threat to the Heat.
In a 7 game series, how long do you really want Lebron banging down low with those two?
To save Lebron's energy, they've had Battier or Haslem guard the big in the post. Haslem's a good defender while Battier hasn't held up as well as he did last year. It's still an advantage for the Jazz of course.
There's a flip-side to this as well. Jefferson is, if not the worst, at least in the bottom five, pick and roll defenders in the league. He'd be a huge liability against a team boasting both Lebron and Wade.