You may disagree with it but mnemonics are very helpful for remembering information that has no relationship to things you already know. I used my own set of mnemonics to learn hiragana and katakana when I first started.

I don't see the merit in using it for Hiragana/Katakana which are far easier to pickup with something like rote memorization.

That said for Kanji I certainly see the benefit of using mnemonics.

That's fine and you're welcome to think that. I would throw the baby out with the bathwater and assume the book is trash because it uses a technique you don't approve think is most efficient.

Different people learn differently. I always found mnemonics useless, but some people preferred them.

The most important thing is to use whatever keeps you motivated. "Keeping on" is 90% of the battle.