See comment above, scrypt was not designed for cryptocurrency wankery it was designed for password hashing, you don't want the attacker to test billions of hashed passwords per second as that lets them trivially brute-force a DB leak or dump. You want costs to the attacker (hash/second/dollar) to be as similar as they are to the regular user, so that the regular user can crank up difficulty as high as acceptable for their use case (e.g. >100ms/password for a web login).
From a cryptocurrency perspective, if custom hardware (ASICs) gives a mining edge, then mining power will concentrate to those who build custom hardware.
If commodity hardware is competitive for mining, then mining power is more likely to be distributed amongst a network of users, rather than owned by a few big players.
You might ask why the network of users doesn't just buy the custom mining hardware. But the economics of it doesn't work out in the long run. If you can build a machine that prints money, why would you sell it to me for less than its lifetime money-printing value?
