As someone who plays Unliving a lot and is usually on the receiving end of this card, here's my take.
As it is now, halving the tower is an excellent deterrent. If I have 120 tower and a bunch of cards that add 8-10, I'm looking at winning in four turns. But if this appears in my opponent's hand, I'm suddenly paralyzed. I could play those cards, but once I get above 140, this is going to knock my tower down to 70. Effectively, each of those tower+ cards really only did 4-5, and I knew it would happen. So, instead, I hold on and play different cards hoping to draw an Uncommon to add +30 or that allows me to target a discard, either of which forces the opponent to play it immediately. The mere presence of the card affects play, and I love that about this. Yes, even as the victim.
Cap it at 50, and this doesn't happen. There's no reason to stop adding to the tower bit by bit. There's no reason for the opponent to keep it for a while. It becomes a boring card.