Also, you are underestimating the new fundamental advantage of Primecoin: it will be very hard to implement it in GPU to be faster than CPU, unlike other coins. The advantage to mine with only CPU, gives many people a chance to mine, taken away from them by specialized GPU mining monsters.
That's a bad thing.
For starters, mining exists to serve the users, not vice-versa.
Cryptocurrencies aimed at appealing to miners are fail by default.
Insofar as POW works in general... you
cannot stop ASICs, period.
It simply is impossible.
With SHA256d, the availability of GPUs and FPGAs made it so the leap from CPUs to ASICs was bearable, and did not compromise Bitcoin's security.
Without the step of GPUs and FPGAs, this leap is almost certainly to be deadly to any cryptocurrency.
In the meantime, before ASICs make sense financially, you have a system that gives criminals (computer crackers) the ultimate authority.
So, to conclude, GPU-resistant is very bad for proof-of-work systems.