I think it was pretty cool for Fire Emblem and fits in with how the game gives you all these different avenues to access free bonus content. Keeps it very fresh!

The first time I saw a dark link I just stopped in my tracks like "oh, hell no..."

Sadly I think it's criminally underused. I take the train to work every day at the big commuter hub for Philadelphia, one of the top 5 cities in the country and my spot pass interactions were kinda few and far between, even for hits like Zelda and FE. I don't think I ever got any data for Etrian Odyssey IV.