Armory can watch an imported address:
1. Make a new (deterministic) wallet on the offline computer
2. Import the private key to the wallet
3. Export the watch only wallet
4. Import the watch only wallet on the online computer
Do you have to do this each time you add a new key? add the priv key to the offline comp's wallet and then export watching only wallet again?
Do I then have two watching only wallets on the online comp?
Y U no have "import watching only key" Armory?
There's a ton of synchronization complexity with importing arbitrary watch-only addresses. You now have a wallet that looks like you have some amount of money, but some of it is in addresses for which you don't have the private key, and now all your offline transactions can't be signed because it keeps creating tx using inputs from keys you don't own. You have to go into expert mode and use coin control, and even then you may have to do a lot of work to figure out what addresses you have keys for and you don't.
The way it's implemented, you must have the private key to add a watch-only address. However, I have thought about adding a special "other addresses" wallet which could be used for this purpose. Just for addresses you want to watch for which you don't have the private key.