TL;DR: No. The argument is basically that hoarding will make Bitcoins so valuable that nobody will be willing to offer people enough to part with them. Does that pass the giggle test? Another way of stating the argument is this, "If gold is $2,000/oz today but people think it will be $5,000/oz next year, nobody will trade any gold today." Again, think about it. Does that pass the giggle test either?
Hoarding increases the value of Bitcoins, increasing the profits from mining. This encourages more people to mine, increasing the total hashing power and thus the security of the system.
It also makes holding Bitcoins more profitable. This helps to encourage people to accept them in trade because they are less worried about them decreasing in value while they are holding them. Using Bitcoins as a currency inevitably means people sometimes have to hold them and having them drop in value while you hold them is a risk. Hoarding reduces this risk. But it also makes it harder to price things in Bitcoins because the value will tend to change more. Merchants don't like to change their prices twice a day.
Contrary to claims, it should not affect the trading volume or the willingness of people to use Bitcoins to pay for things.
The argument that increasing value means people would prefer to hold Bitcoins rather than spend them is specious. While it will make people want to have Bitcoins more, it will also make people want to convince others to give them Bitcoins more so they can have them.
Think about it, do people prefer to pay for goods in dollars or garbage? By the reasoning of this argument, they should prefer to pay for goods in garbage, since they'd rather hold their dollars and get rid of their garbage. But, of course, people don't like paying in garbage because nobody wants garbage. If you can pay with the currency others want, you can get a better deal. So you actually prefer to spend the currencies sellers most want.
If Bitcoins are valuable because inflation doesn't deprive them of value, then a merchant would rather get my Bitcoins than my dollars, so he'll accept fewer of them. This will cancel out the effect of me preferring to pay in dollars rather than Bitcoins. So it should be a wash.
Or, put another way, whatever the present and future prospects, there should always be some equivalence between dollars and Bitcoins that people roughly agree on. So whether I pay X dollars or Y bitcoins, where X and Y are in this ratio, will purely depend on whether I prefer the characteristics of dollars or Bitcoins for the transaction.
A consequence of this is that hoarding won't negatively affect the trading volume either because that's only dependent on people's use of Bitcoins to buy and sell things. If hoarding makes Bitcoins worth twice as much, only half as many will be used to buy and sell things and the volume (the total value traded) will be the same.
I don't believe it's lost opportunity either. It's not like people who want to trade Bitcoins can't get them because they're all being hoarded. (Nor will it ever be likely to be an issue, since that would just raise the price and thus fewer would be needed. Bitcoins have effectively unlimited divisibility.)
So why do so many people think currency hoarding is bad? Because it usually is, and empiric studies even show that it is. But the logic of why currency hoarding is bad doesn't apply to Bitcoins, especially when it's a minority currency.
Bitcoins are different enough from physical fiat currencies that empirical studies don't apply unless the suspected mechanisms for the observed effects are believed to apply to Bitcoins too. For example, if a penny were enough money to buy a car, how useful would dollars be?
