Why? Because if a merchant accepts both Bitcoin and Fiat currency people will always prefer to pay with fiat currency simply because the equivalent bitcoin is seen to be more valuable (i.e. $100 woth of bitcoin is > 100$ fiat).
I think this is an interesting point. Thoughts?
It's nonsense, for two reasons:
1) If it was correct, it would apply equally well in the other direction. If a merchant accepts both Bitcoin and fiat currency, people will always prefer to pay with bitcoin simply because the equivalent bitcoin is seen to be more valuable and thus the merchant will be willing to accept a lower price.
2) It contains the oxymoronic claim that that the "equivalent" bitcoin will be "seen to be more valuable". If it's seen to be more valuable, it's not the equivalent, it's less than that.
The reason people hoard gold (and people may hoard bitcoin for the same reason) is that they irrationally believe it will increase in value (or, equivalently, that their fiat currency will decrease in value) more than other people do. So they think others are underpricing its future value today. However, there are also going to be people who think bitcoin will blow up any day now, so they will think others are overpricing its future value.
In any event, hoarding is a fact that influences the price of a currency, but that influence is to increase its value. Scarcity isn't a problem with bitcoins because they are divisible.