Buzz-saw is far from worthless, but the damage difference makes it hard to commit to that variation.
The pressure in buzz-saw is quite good and it has much better oki than tempest. Also, ex spin in buzz-saw is much better in the neutral than its tempest counterpart (though the opposite is true when it comes to anti-air). Buzz-saw on the whole is actually better in the neutral, and low hat gives you another tool in KL's already great neutral game. Forcing the opponent to crouch or making him press buttons and do something stupid like jumping or something like that is a pretty big deal.
Low hats on hit gives you continuous pressure and they're safe on block, and unlike tempest, you don't spend meter for your pressure.
Of course, this is only looking at the positive side since the advantages tempest has over buzz-saw in other areas are clear, but I'm just disputing the notion that buzz-saw is worthless. It really isn't.