As far as I know
all wallet app have this shortcoming. It's not easy to overcome... unless you scan the blockchain on every load
Because even if you encrypt the watching-only wallet, any data you save between loads can be used to identify what money is in your wallet.
This was one of the benefits to having the super-node version of Armory that maintains a full DB of all spent and unspent UTXOs, so that it can encrypt the watching-only wallet without leaving leaky information all over the disk. You just decrypt the wallet and instantaneously request all the address balances from your local database (among other uses for the super-node version). But the super-node stuff will be put on hold for now. Got some other priorities.
What I hope to do is get the blockchain scanning in Armory faster (I missed a lot of database optimizations in 0.90), and then have an option to disable saving data between loads. Then it will behave much like 0.88.1 and earlier where it scanned every load.
Then it makes sense to implement the encryption.