I don't think entropy is the whole story. I would argue that although security-through-obscurity is a terrible, awful practice for systems, it's not that bad for personal password schemes. Using a nonce "system" for passwords, even if it's mathematically low-entropy, is still secure, at least enough for personal use.
For example, if I use single dictionary words fed through a trivial ceasar cipher, then that is mathematically very low entropy. Realistically speaking, however, it's relatively safe if the cracker doesn't know that's what I'm doing, because it's impractical for crackers to compute all possible low entropy "alternative dictionaries."
You'd be surprised. JTR does l33tspeak substitutions, one-row-up substitutions, keyboard walks, pretty much all of the common things everyone does because "no hacker would ever think of that."
For example, if I use single dictionary words fed through a trivial ceasar cipher, then that is mathematically very low entropy. Realistically speaking, however, it's relatively safe if the cracker doesn't know that's what I'm doing, because it's impractical for crackers to compute all possible low entropy "alternative dictionaries."