Yup. It's nasty.....but only especially so at low (for this EL) ACs.
I just ran the numbers for the three possibilities:
Pit Fiend: 2 claws, 2 wings, Bite, Tail
Pit Fiend w/ longsword: longsword (4 atks), Claw, 2 Wings, Bite, Tail
Pit Fiend w/greatsword: greatsword (4 atks), 2 Wings, Bite, Tail
Let's pick two representative ACs at this challenge rating (CR 20). IME, AC 30 and AC 40 are in the ball park.
Code:
Ave Dam/Full Attack AC 30 AC 40
Pit Fiend: 105 54
Pit Fiend w/ longsword: 118 50
Pit Fiend w/greatsword: 145 60
(Here's hopin' th' table works....)
As you can see, at higher AC, the advantage for the weapon becomes less. Still, it's a good option, and we're not even making the weapon magical yet, or slapping a few levels of Ftr on the silly Pit Fiend.