I want the set to feel different from Conspiracy, but still have areas with politics in mind. Hostage is that area - note that the reminder doesn't say "you control", as it may be helpful to save one of your opponent's creatures in order to bring down a common enemy later. Hostage will be in White and Black, with one or two cards in Red and Blue. Also note the "Target player" - there will be a couple of cards (probably not a mechanic) that will care about having the lowest life total, so this is also in support of that.

Does this really need to be a replacement, instead of an activated ability like on Fanatical Devotion or Vigilant Martyr?

It makes it a bit more strategic to play against, because you can remove the Hostage before combat damage. It's also slightly different from Regeneration in that you can remove the creature after blockers are declared to get rid of its "shield". But from a purely design standpoint, not really.