That argument also works against anubis. Why bother forcing clients to solve a PoW challenge, when you're fighting a bunch of unsophisticated attackers that will be deterred by a javascript challenge that requires them to solve `result = x + y`?
Turns out that in practice there's some bots that implement support for this but not JavaScript.
People have been sent to jail for participating in LOIC attacks which were less destructive that what these bots do on the daily.