Just because 50 NMC took just as much to mine as 50 BTC doesn't mean that anyone's willing to spend 50 BTC on 50 NMC (one domain name) - that's a ridiculous price for one domain name (and nobody uses NMC to buy anything other than domain names at this point). If you have one NMC you're going to have to sell them for a lot less than one BTC, if anyone is going to buy them from you.
There is no reason to believe that 1 NMC will be equal to 1 BTC due to the perceived value of the two. However, since both are mined at the same time, there's reason to believe that the exchange rate for both will become mostly fixed to say 10 NMC per 1 BTC, which essentially means the 21m coins cap for BTC has just gone up 10%. If 1 BTC were considered a dollar, an NMC could be considered a dime. Add in more currencies and the cap will go up even more, leading to an inflationary currency instead of a deflationary one that many were hoping for.