^ He never argued that arbitrage is unhealthy. I think you misunderstood his post. He's saying that having multiple BTC-based blockchains doesn't better enable one to take advantage of fiat-BTC arbitrage opportunities.
Ah, his use of the word "combated" leads me to believe otherwise. Seems you're right about the fiat-BTC arbitrage, though. In any case, the BTC/USD rates of e.g. btc-e and vicurex are typically a lot closer than btc-e and mt. gox, presumably because there is alt-chain enabled arbitrage, so hopefully it's a non issue.
The rates are closer presumably because the friction of getting fiat into and out of btc-e is closer to what can be found in vicurex than MtGox. I don't see any reason why BTC-altchains would enable BTC/USD arbitrage, for the reasons
provided.