I agree with many here that the number of legendaries in later generations is too high. Legendaries can popular among kids and their stats can make you feel invincible in-game, but it feels as if some were added as a obligatory gimmick rather than because the developers have a grand idea behind it. Trio, Trio, another Trio, yet another Trio (how about a different number, 4?) Gen IV had so many legendaries covering so many concepts, yet there isn't much impact (to me) since every pokemon is trying to steal the limelight. Time/Space/Anti-Matter + Knowledge/Emotion/Willpower + New/Full Moon + Sea guardian + Earth's Core. Seriously, the last few could have been a generation mascot on their own with the right pairing and ideas.
Sometimes, it's the quality and not the quantity that matters. It's not like Game Freak is going to end the franchise soon, so why rush with the ideas?
I wouldn't mind a few pokemon being introduced like how Magma, Electabuzz, Snorlax, Sudowoodo were in Gen I and II, though. They were part of the plot device or had some sort of storyline, and you only have a few chance to catch one, but now you can breed more.