Saying that the value of NMC is linked to BTC isn't a sane argument as to why this shouldn't be implemented.

The only reason that the difficulty is linked to the exchange rate right now is because I have to choose between mining NMC and BTC.

As an example, let's say the difficulty of NMC is 1/2 the difficulty of BTC right now.
If the exchange rate is more than 2 NMC per BTC, and I want Namecoins, I'd be better off mining BTC and exchanging them to NMC (Thus lowering the exchange rate).
If the exchange rate is less than 2 NMC per BTC, and I want Bitcoins, I'd be better off mining NMC and exchanging them to BTC (Thus increasing the exchange rate).
And so because I can choose between mining NMC and BTC, and I will always do what's the best deal for me, the exchange rate is directly linked to the difficulty.

When I can mine both BTC and NMC at once, the linkedness falls apart completely. Suddenly, if I want BTC, it's best I mine both and exchane the NMC. If I want NMC, it's best I mine both and exchange the BTC.

So now everyone's mining both, so the difficulty becomes equal, and since the miners no longer look at the difficulty and exchange rate of both, and they are no longer linked by difficulty, only by how many people want BTC and how many want NMC.

Since BTC has caught on much more than NMC, I predict the exchange rate for NMC will drop a ton because all the Bitcoin miners will suddenly have Namecoins and want to get Bitcoins (and while the same thing will happen for namecoin miners, there are much fewer of them).