This was brought up and discussed at length in the cease PPC/NBT operation thread. Most people seems to agree that idealy NBT/USD should be the only pair Nu support but due to initial adoption rate and liquidity NBT/Crypto should be offered. As a redult a tiered bot order book has been proposed to reduce risk (ala “junkcoin”) and hedging is recommended to be used to mitigate exchange rate risk.
I think OP is still a timely discussion in light of increased adoption of Nubits and increased volatility of cryptocoin coin price lately.
Thanks @benjyz for pointing out the importance to separate market maker, arbitrageurs, and the exchanges, and that marketmakers should be allowed to set spreads.
Back to the OP, I suggest we also look into addting Nubits to direct purchase option of payment processors and gateways.
For example you can buy bitcoin/litecoin/peercoin at https://coinomat.com/#PPC/OKPAY and have the coins directly sent to your address. The exchange rate is pretty good. Why not get Nubits there? There are many more examples such as https://xchange.cc/en/ . OKpay supports BTC, LTC, DOGE. Payment processor giant Perfect Money handles cryptocurrency and offer cryptocurrency wallets. NXT Assets&Money is a certified partner of Perfect Money. Can Nu have something similar?