Re: Why have mana leech on a caster helm?
It's potentially useful if you were trying an untwinked/single-pass melee character (something like an Avenger). Considering you could have the helm by Act3-4, it may be the most reliable early-game mana leech there is for untwinked/single-pass. Pyrohemia's point about buying mana potions is valid too.
<--- Not exactly the most common scenario, but as stephan said, there isn't a lot of rhyme or reason to the pre-set stats on a lot of items. This one isn't really any worse than many.