Why people would sell you something for bitcoin? Because they believe that later someone will buy that bitcoin from them.
And since a bitcoin is not a directly serviceable good, that is the *only* reason to buy bitcoins. It's commonly known as the "greater fool theory":
http://en.wikipedia.org/wiki/Greater_fool_theoryIsn't it nice to know that the entire "bitcoin economy" is based on everyone expecting to find a greater fool to buy their bitcoins?
You seem to be very emotional about this - but call it as you wish - this is the same thing with all money and with gold in particular. The use value of gold is just a little fraction of it's exchange value - so why people buy it? Because they expect someone else to buy it back from them.
Bitcoin is a currency - how it emerged is not relevant for the definition. If it will survive is a completely different question - lot's of currencies are dead now.