It's not just about purpose.  As a real world example, take the USD.  It serves a purpose and has been the reserve currency for over half a century.  If they start printing a lot more money, would you rather someone paid you in USD or a currency that doesn't need to go through major devaluation like AUD?

Also, not everyone believes in NMC.  Anyone can come up with blockchains with a "purpose" that a niche will enjoy mining in.  When mining in 100 currencies costs the same as mining in 1, people won't care that they're mining in a currency that's used to pay for videos of someone torturing insects.  If you're not for that cause, someone else will be and you not mining in a pool of only 20 currencies with ideas you believe in will cause you to earn much less than the people that are mining in all of them.

Remember once NMC and BTC are both mined at the same time, the exchange rate between the two will eventually be fairly constant.  It'd be like the BTC was considered as a USD $20 and an NMC a USD $1, effectively increasing the money supply of the base currency (BTC) and devaluing both.