It may lead you to believe that 40 power x 2 hits is the same as an 80 power hit move, but it's not.
Let's use some logic here:
Let's say that Dual Chop does around 50 - 70 damage per hit, meaning two hits is around 100 - 140 damage. Now, let's say that Dragon Claw will have a range of 120 - 130. Because Dual Chop hits twice, each move's power is going to vary slightly, as (excluding moves like Dragon Rage and Sonic Boom) no move has set damage. These means, because of the variation, the possible range on damage is either much either or much lower. It has a higher max damage than Dragon, but a low minimum damage. This is all because of how much Dual Chop's per hit damage can differ. In this case, Dragon Claw is going to be doing more consistent high damage, because it's only one attack.
Dual Chop can be useful, especially going against things like substitute or Sturdy users.