Well, there are different board games too. For example in Chess (Yes, I consider Chess a board game. You play it on a board, don't you? lol) the goal is very clear: Win against your opponent. Of course that's just one opponent, so fine, let's up the ante. Chinese Checkers. You're still trying to get all your pieces to your opposite point before everyone else. Clear victory conditions.
BM is supposedly not like that, at least on the large scale. People insist that you can't "win", though what exactly do they mean by that? Obviously you can win against enemy realms in-game, but there is no overarching victory condition. In that case it's somewhat like Scrabble, depending on how you play, where sure, you can tally points, but any "victory condition" such as "Get to X points" would be arbitrary and not a feature of the game itself. That, I believe, is more or less where BM fits. You just keep battling it out, or something...Not sure. I don't recall ever seeing pity moves in any board games I've played though.
Chess? Get mercilessly destroyed. Ok, sometimes depending on opponent he'd toy around with me before the final blow. Monopoly? One does not stop to let his competitors breathe when he attempts to fulfill his destiny by achieving the game's namesake status. Risk? Strategic allowance of existence before you take everything.
Maybe that's where BM also fits in terms of mercy play. It's like Risk, where it would be of benefit to you to let the realm live. But in Risk, the victory condition is most often "Conquer Everything". In BM, I'm pretty sure you can't conquer everything. But if the goal is just to survive, well, that's actually not so hard. Just be so insignificant no one would care to fight you. But if it's to have a future ally, well, ok, that could work. Things like that are complicated, sure. I understand that. I'm not against those. Though there are some cases that at first don't make sense. Yeah, sure, there's always a story behind that stuff.
Now let's pose the hypothetical and consider exactly how much benefit would arise from keeping the realm alive versus the consequences of destroying it. Sometimes the skew is not that extreme, in which case, well, destruction does let you fight a new enemy, sort of. You get to fight a realm with a different name (Or in Tuchanon's case, the next Roman numeral appended to the name). Maybe they're the same guys, but eh, whatever, new faces, new enemies to defeat and all.