I don't see the point of encrypting a wallet.
It wouldn't be compulsory. If you don't want to encrypt your wallet then don't.
How strong is your encryption key (pass phrase) going to be?
It's going to have over 100 bits of entropy, making it impossible to brute-force. Hopefully the encryption will use something CPU-intensive like scrypt, making brute force attempts less likely to be successful for shorter passphrases too.
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.
You will have it written down on a piece of paper, because a long, random and rarely used secret is easy to forget.
I don't write my passwords on paper. That seems like a bad idea. I find that typing all my passwords at least once per month is enough to keep them in my memory.
Cold storage is simpler and more secure than encryption.
I've not tried using cold storage with Mycelium. I prefer to keep my cold storage keys completely offline. How does it handle coin selection and change addresses? That seems like it would be a pain, if it's even handled at all.