With a brain wallet, the key is memorized. There is no need to encrypt it, because nobody has access to it. Anyway, encrypting it would make it impossible to memorize.
Not necessarily. Some people use parts of books, personal stuff or just picked words to generate their own brain wallet "custom seed". This may not be the safest thing, but some people do it anyways. Adding a password would be like a second part of the seed. If you use a part of your favorite book, you can then add a quite-easier-to-remember password (like mike123) to "protect" against someone finding the seed. Does that make any sense?