1. They were actually plot relevant when they were introduced in GSC, tying in with the legend of Ho-Oh. I felt as though the legendary birds being connected to Lugia was a shoe-in, which didn't factor in to either RBY or GSC's stories. Only the anime went in-depth with them in a movie.
2. They were themed after the three wildcats: lions, tigers, and leopards/cheetahs. These animals are usually thought of as trio to compare to in real life.
3. Roaming legendaries were a brilliant idea. Some people thought they were ridiculously difficult, but face it, shouldn't legendaries be?
4. We're given a backstory for these pokemon, along with reasoning for their types--the tower was struck by lightnining into fire on a rainy day. As for the birds, they're just... there.