Would it be useful to have a client that only allows you to import
private keys into an encrypted wallet for the purpose of monitoring your Bitcoin trade value? This would be for people that store their bitcoin in physical form and don't want to spend it.
This is already possible with Armory. You create a full wallet that has the private keys (which might be created on an offline computer that never touches the internet), then "Create a Watching-Only Copy" of it, which doesn't contain any private keys. You can even import private keys into the full wallet before you fork the "watching-only copy" and those addresses will be monitored the same way. Import that wallet into Armory on your online computer, and you can watch all the addresses without having the private keys exposed to the internet.
If you only have one computer, you could create the full wallet, print a paper backup, then delete it from the computer and keep only the watching only version. However, this would make it super-annoying to spend any of that money, requiring you to re-import that wallet from paper every time (though, what other options do you have if you don't want to keep the private keys on any computer, anywhere?).
What I don't have, is a way to import public
keys directly into a watching-only wallet (only private keys into a full wallet). But that should be an easy upgrade if it's a high-demand feature. But, in the end, the private key has to touch some computer
at some point, unless you plan on calculating the public key and address from the private key, with pencil&paper.
The beauty of Armory is that the watching-only wallet, will be able to generate an infinite series of [deterministic] keys, that can be used to receive Bitcoins, and the addresses it generates are exactly the same as are generated by the full wallet (this is described by gmaxwell as a "type-2 deterministic wallet"). This allows you to manage all your money from an online computer without the private keys, and
use new address for every incoming transaction. The only real risk is that, if the online computer is compromised, your privacy is breached but the security is not (they can't spend your coins, but they can link all the addresses used by that wallet).
If you want to spend the money in these addresses: well that's what the "Offline Transactions" interface is for -- allowing you to shuttle transaction data between online and offline computers using a USB key, and the private keys can stay offline. And Armory will walk you through the whole process. See Using Offline Wallets in Armory
for more information (illustrated tutorial coming soon!).
I think that completely covers the use-cases you asked about. If there is a feature missing from this description, please let me know, and I'll see how I can accommodate it in a future release! EDIT: I realize now that you were talking about "trade value." I assume you are talking about grabbing MtGox+Tradehill price for your local currency, and displaying the balance of your wallets in that currency...? That would literally be a few lines of code added to Armory to query the prices online, and then replace the "Balance (BTC)" field with "Balance ($)" field. After that, everything I wrote above is applicable.