It's not worthless in the sense that you can't find a sucker to sell it to today. It's worthless in the sense that eventually the supply of suckers will run out, and you're running the risk of getting stuck with bitcoins that have no other value. (Of course the same thing can happen with national currencies...but if it does, it means that the nation that issued the currency has collapsed, and if you're a resident of that country then you've got bigger things to worry about. Even a solid gold-based currency might not be tradeable for food or medicine or weapons.)

It helps if you notice that it's like every other pump-and-dump scheme:

Con Artist 1: I'll buy a bitcoin from you for $1.
Con Artist 2: OK, here it is. Actually, I think I'll buy it back from you for $2!
Con Artist 1: OK, here it is. Will you sell it back to me for $4?
Con Artist 2: Absolutely!
Victim: Wow, the value of bitcoins has quadrupled in the past few minutes! They seem like a wise investment! Can I get in on the action?
Con Artist 1: Sure! In fact, I'll sell you as many bitcoins as you want for only $3.50 each. That's below the market rate.
Victim: What a bargain! I'll take a thousand!
Con Artist 1: Done!
Victim: So, who wants to buy bitcoins? The bidding starts at $8 each!
Con Artists 1 & 2: My, my, look at the time! We must be going!