Sure, if you have arbitrarily large mana or an arbitrarily usable blink engine, it will keep everyone from attacking always, but at that point, Stonehorn Dignitary isnt the problem
Defeat four of your opponent's creatures, and you can transform Sephiroth into the One-Winged Angel, which only improves Sephiroth's already excellent abilities
If you hold back a hasty creature or plan to activate a creature land like Den of the Bugbear after the wipe, that can keep the pressure going even after your opponent resets the board
Sure, they have white borders
Some aristocrat or white weenie decks use to put this card to good use in Standard
Thankfully, with Wilds of Eldraines Enchanting Tales containing powerful white reprints like Land Tax and Smothering Tithe, Moonshaker Cavalry should have all the support it needs to have maximum impact and regularly close out games