New Phyrexia is the home of the Birthing Pod, a Modern staple removal card (Dismember), one of the best Equipment cards of all time (Batterskull), and some really sweet creatures such as Elesh Norn, Grand Cenobite. Of course, we can’t forget about Phyrexian Obliterator.
When this card was first released, Phyrexian Obliterator plus the powerful equipment Lashwrithe in mono-black decks in Standard could equal "Good Game." Of course, there was also a little uncommon in New Phyrexia called Deceiver Exarch that made an old bulk rare called Splinter Twin a fairly good win condition.
Yeah, mono-Black infect was pretty sweet, and having the Obliterator around to hold the ground was pretty awesome. Sure, 4 Black mana symbols is a rough casting cost. But, that’s not a problem in Mono-Black, even when you were running 4 copies of Inkmoth Nexus - one of the dumbest lands ever printed...
Still, Phyrexian Obliterator was a $25 mythic rare that no one seemed to want. He was stuck in some Birthing Pod decks and saw some Legacy play. Really, though, when you looked at him, he looked like a 4-drop win condition with 5/5 power and toughness & trample. Plus, he has that nasty Annihilator-type ability whenever he’s dealt damage.
Heck, you could use Darksteel Plate on this guy and he’d be an indestructible force of destruction. Dismember and Go for the Throat (and perhaps the occasional Day of Judgment) were really the only ways to take him out. And, with a Lashwrithe equipped, you were talking about at least a 9/9 creature with trample, and likely more.
As time has passed, Phyrexian Obliterator is still a really nice card to have. He’s found homes in many mono-black EDH / Commander decks. Modern decks have occasionally found room for him, with variants of Birthing Pod (until it was banned in Modern) and even some Jund brews utilizing him.
Mono-Black Devotion, however, is the deck that would seem to be able to use him the most. You make Erebos, God of the Dead active and make Nykthos, Shrine to Nyx relevant quickly. So, why has no one built that deck?
While it looks awesome on paper, the deck’s not quite good enough to compete with the rest of the format, it turns out. Path to Exile exists to wipe him away in that format. Obliterator just doesn’t last long in the format, and relying so heavily on his 4 Black mana symbols for the deck to fire off is a risky proposition.
Still, he's a pricey card, even with a reprinting in Masters 25. That’s because he’s a mythic rare from two highly sought after sets. There are a ton of good mythic rares in both of those sets, and especially in New Phyrexia. Phyrexian Obliterator is a good card – he just doesn’t have much of a home in competitive play.
Still, Obliterator is relevant in Commander and he is very relevant for Devotion. There is still potential for the Obliterator to sneak his way into a Top 8 Modern deck in the future. Is he worth the $20-25 that you’ll have to shell out for him? I don’t see why not. It’s likely he won’t see another reprint anytime soon and both New Phyrexia and Masters 25 are becoming very rare to find. He’s a pretty safe investment, and he can always find a home in many mono-Black Commander decks. So, he’ll always have demand.