If your pass phrase is long enough to be secure, do you really want to type it in every time you need it?
Yes, that's exactly what I want. I want to be able to unlock my private keys for a specified amount of time by typing the passphrase. The same as Bitcoin Core does. I spend from Mycelium maybe once a month. I can handle typing my passphrase that often. I type it into Bitcoin Core more often than that, and it's not a problem at all.
Use BIP38 encrypted paper wallet, which you can spend from using cold storage option in Mycelium. Change goes back to the paper wallet address.
Two feature suggestions:
1. Some audible indication if you receive bitcoins. Andreas Schilbach's wallet has a very neat 'tinkling coins' sound. Just something subtle. Would be a great addition in the user experience, I think (sounds crazy, but often I've heard "damn I love that sound of incoming coins!")
We keep getting asked this, and we keep having to explain that this is terrible for privacy. The only way a wallet can know that you have received coins is to constantly keep asking a node for an address babalce. Whoever runs that more then knows that that's your address. And with HD you would be pinging multiple addresses, so the node will know they all belong to you, and will be able to piece together your entire transaction history.
By the way, for you guys in Europe, check out Mycelium's Cashila integration. I can't use it since I'm in US, but if you register on Cashila and register your Mycelium wallet with your account, you can send money to any IBAN/SWIFT account directly from the wallet (to pay bills or transfer cash into someone's account)