Yes they have value because they are desired.  Tulip bulbs were valuable because they were desired, and desired because they were valuable.  My problem is that if a thing is ONLY desired because other people want them, then no rational price exists.  Tulip bulbs could be $100 or $1000 or $10,000 and none of these prices is any more rational than any other price.

Actually, there is a rational price, which is the commodity value.  If it has no commodity value then it is zero.

Even if the rational price is zero, it can still have numismatic value.  Old stamps have value because people want them, and people want them partly because they have value to others but also because they derive joy from owning old stamps.  I'd say this is not rational value but it is value nonetheless.

In the case of bitcoins I think people derive a similar kind of non-rational value from believing that they are escaping the system.

"I spent $1000 on this rare T-shirt as an investment and all I got was this lousy T-shirt"