The idea of the build is avoiding jungling items so you can start building for mid to late game faster, and ganking a bit earlier with better preparation. If you're planning hardcore jungling then you should stick to cloth armor into wriggle's.
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I would argue it's efficiency due to it being just as safe as armor + 5 pots while wasting less money on health pots at the start. I also feel mana crystal is generally more useful for mid to late game items ie: sheen-> triforce, catalyst->banshee's veil.
There's also the added benefit of having full health and mana as well as red buff at 5min. This is great for ganking, but also because you can easily go back into laning without having gimped items and while having the buff advantage, and while having a lane partner that outlevels the enemy.
One of the most annoying things about Udyr is his early game. His spells cost too much mana until they're L3+ and his mana pool is tiny. This build lets you blast through that phase while putting him in a good position for the midgame. Most of the time, jungling feels like it closes up options, but with this method I really feel like you only have options opened up to you.