Forget about the PoH Fire staff, it's completely useless as you will not want to use a fire pet post 50, and DDD is not focused by Staff of Elemental Mastery : Fire
Phinny can be taken down by a couple pet classes + cleric or 3 mages/necros, and he's a 12 hour repop.
Plane of Sky staff is not that hard to get, more so since it is now possible to MQ any PoS quest, and I can guarantee you a focused max level air pet is a stud.
Line forms behind me for the PoH Earth Staff...
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Even non focused 57+ (Greater Vocaration: XXX) pets are awesome.
You'll get some nice nukes,
handy utility spells (useful debuffs, Mod Rods, WR bags, CotH of course),
kick ass pets that give you a very steady dps compared to other classes who only rely on their mana pool,
and as some already mentionned second to none green farming ability (think Droga goblins, HGs, spider silk, velium pieces/ blocks, etc.)