Intrinsic value is set by the marketplace. Right now, the market perception is that prime numbers (or whatever bitcoins are) are scarce, valuable resources.
Bitcoins are not simply prime numbers. They are account balances in a system that distributes the validation of account balances in such a way that is cryptographically secure, trustable, and decentralized. Provably correct, tamper-proof accounting balance sheets are nothing to sneeze at and it was really a stroke of genius to discover the algorithmic means to create such a thing that is the cool thing about Bitcoin.
I personally believe that it will never achieve anything other than niche status as a currency for trading illegal goods; it will be used for other types of transactions, but it will be a very small number of transactions when compared to contemporary financial transaction systems like, e.g. credit cards, or paypal. I say this only because in practical terms it is not very easy to use, and furthermore, its benefit of being pseudoanonymous is also is downfall; very safe for the recipient to receive bitcoins but very risky for the sender to send them since the sender has absolutely no insurance or any recourse in the transaction. There is no authority to appeal to for transactions gone bad, and even if you could sue over bitcoin transactions and the courts would understand them, the pseudoanonymity would make it very difficult to know who to sue.
I don't think Bitcoins will achieve prime status either, but not for the reasons you listed:
- Easy of use: this is mostly a matter of software, and I see no reason to believe it has to be difficult to use. With current proposals such as specific tags for websites and such, sending money can be made as easy as inserting someone's email address, the amount and hit "send."
- Unsafe for the sender: nothing in the design of bitcoin prevents the creation of escrow services. It's simply not an issue to be fixed by the currency but by higher level services, just like with other currencies.
- Pseudoanonymity: it's mostly a red herring. When I'm buying from a random Chinese 'shop' through Ebay I won't be able to sue him either. When I'm buying from a reputed shop in my country, I don't need to ask its bank to know who I need to sue.
