It works in one type of deck, a Necrodeck. However you remake it, it's still a Necrodeck. Necropotence is not just a card you throw into a deck because it's just so wonderful. Phyrexian Arena isn't like that at all. All it is is extra cards for life.
Any deck can use that, no matter what colors it is.
Even just having one out gives you advantage over your opponent. Any deck using black will find it useful. It's not as explosive as Necro, no, but in the type of environment that's Type II these days, you don't need to be that much faster than your opponent to win, you just have to stay ahead of them, and more cards in hand is the way to do it. Arena won't fill in the Necropotence slot for all those Necrodecks you have sitting around the house, but it will inspire a few new deck ideas, and find it's way into decks that never could have used Necro to any advantage. Especially with the ability to 'stack' multiple copies and therefore multiple draws.