This is something I have missed ever since Time/Darkness/Sky that is also not in GTI: Friend Areas. In Red and Blue, to recruit a legendary, first you buy it's insanely expensive friend area, then go through a very tough dungeon (usually an agonizing 99-floor dungeon). Now, you just stroll into the dungeon and get the legendary. Without any friend areas, any ol' fool can walk in and have legendaries by the dozen. Oh, and most legendaries can't be recruited.
3DS FC: 4597 - 0384 - 5442
If you add me, PM me with your friend code and I'll add you.
Friend Safari: Grass; has Oddish, Ivysaur and Quilladin