The danger is if people are buying bitcoins in the expectation that the price will go up, and the resulting increased demand is what is driving the price up. That is the definition of a BUBBLE, and as we all know, bubbles burst.
It could be a bubble, on the other hand if the increase in price, even if driven purely by speculation, encourages more businesses to accept bitcoin as a form of payment that can serve as a real foundation for a bitcoin economy.