Guess they thought it would be all wine and roses and looted booty after they busted up Silk Road and arrested the Dread Pirate Roberts. Now they're finding out that they have no way to access the bitcoins they seized from the members of Silk Road or Roberts. There's a lot a money there and, unless they know how to decrypt the wallets they'll never get their grimy hands on it. LOL. Modern tech has finally come up with a way to stop the government from enriching themselves by seizing money. It may not be usable by its true owners, but the FBI and .gov are never going to see a penny of profit off of it.
Ignorant FBI Agents Realize They Can't Open Bitcoin Digital Currency Wallet | The Libertarian Republic
Most likely they want his wallets because they contain transaction information (who/how much) for transactions coming and going. With that information, combined with the database of the website, and who knows what else, they can begin to figure out who is making/buying the drugs.
The value of that money is all theoretical and fluctuates daily. If they were so concerned about the bitcoin stability they could just cause a few of the exchanges to go offline -- last time that happened Bitcoin tanked.
No, I doubt they care about the "value" of bitcoins and care more about getting to the makers/buyers of the drugs and access to the wallets would help make that easier.