Armory has a multi-wallet interface precisely because it
does support different security models. Many users keep different wallets also for general organizational reasons, and I've heard that the bitcoind "accounts" are very ... awkward.
We spent a lot of time making the interface natively support multiple wallets, because having a single wallet doesn't meet the needs of most users -- you need to be able to manage "spending cash" (hot) and "savings" (cold) simultaneously, you can't just keep all your money in one or the other (you can, but it's not ideal). Also, we use watching-only wallets to track employees payments. For instance, here's why we hold multiple wallets:
- A hot wallet for day-to-day spending
- A watch-only wallet for an offline/cold laptop in the office
- A watch-only wallet for an offline/cold laptop in a safe-deposit box at a bank
- N watch-only wallets for each N employees, so we can pay them without requesting or reusing addresses. Armory also has an export function to save the transaction history for each wallet to a CSV file for book-keeping.
The last one isn't limited to companies. You and your friends can exchange watching-only wallets, and use them to pay each other. It's automatic book-keeping, easy backup, and never re-uses addresses.
Trying to deal with multiple wallets in any other app is kind of a pain, and not all that useful unless you're using significantly different security profiles for each one. I agree that if the private keys for multiple wallets are on the same hard-drive, they have essentially the same security profile (assuming they're both encrypted).