http://www.npr.org/blogs/thesalt/2011/11/02/141771712/how-fe...
The perception of scarcity, which leads to hoarding, only makes the scarcity problem worse for everyone.
Hoarders/speculators help by smoothing the scarcity and resulting price increases over time. By buying things up early, they increase the price and scarcity early, which gets people motivated to switch to alternatives when they can (and warns people who weren't paying attention to whatever caused the shortage, the floods in this case, of the problem). When they resell later, their stock keeps the shortage and price increases from being as extreme as they would otherwise get without the hoarders.
For one, the shortage has to be a real one, and not directly caused by hoarders, as happened with toilet paper during the 1973 oil crisis. In this case, the hoarders pretty much hurt everyone (other than providing a very clean and clear example of the Thomas Theorem).
For another, the hoarders actually have to sell around the height of the shortage. If they sell after the shortage is resolved, then they took what they were hoarding off the market only to resell again when it was no longer needed and probably took a loss in the process.
So, hoarders can certainly help smooth out a shortage, especially an unexpected one, but it will not always work out that way.