The term 'legendary' is very ambigious and not very well defined, kind of like 'Pokemon master'. Every new generation breaks old fan-given 'rules' for what a 'legendary Pokemon' is:
Gen II: legendaries no longer 'required' to stand still and be battled, they could appear in the wild (legendary dogs).
Gen III: legendaries can have genders (Latis) and be version-exclusive (Kyogre and Groudon).
Gen IV: legendaries can have variable genders (Heatran), events can be associated with non-legendary Pokemon (Rotom), non-legendary Pokemon can have
changeable forms (Rotom), non-legendary Pokemon can have 'legendary Pokemon' music (Rotom), legendary Pokemon can breed (Manaphy and Phione), two legendary Pokemon can be in the same evolutionary family, for lack of a better term (Manaphy and Phione) and legendary Pokemon can have low base stats (Phione).
Not to mention in the anime Unown are clearly legendary. Meaning that the term varies by canon.
So, this brings me to my point, what's stopping these from being called legendary by fans:
In my opinion, Nintendo should make a certain few Pokemon, that are only available via Events; and not as many Legendaries!
@Iyo - I know, but they should just have a rarer Pokemon in their place or JUST ONE.
And it also begs the question, what's wrong with legendaries? Is ithat people call them legendaries, is it that they too powerful (in which case, you could be saying they should just introduce weaker legendaries, like Phione).
tl;dr legendaries aren't clearly defined.
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Personally, I like legendaries, they're great sidequests. If there weren't all these legendaries in Platinum, we'd be left with, finishing the Pokemon League, making our way to the Battle Frontier and bam, that's it, no other secret locations or hidden sidequests.
Anyway, as I said, the number of legendaries is ridiculous. They've gone from being interesting and exciting additions to the game to being frivolous novelties. Back in first gen (and, to similar extent, second gen), it was a big deal to catch a legendary and it made you want to go back to get the rest of them. In Diamond? I accidentally stumbled upon Giratina. I didn't even know it EXISTED until suddenly, I was facing it. :| Maybe that's how some people prefer it but when I start stumbling upon legends I didn't even know about (especially when I thought I'd already encountered all or most of them!), then we have a bit of a problem. ~_~
I do prefer talk about the legendaries to be done beforehand. Obviously Giratina was just added ready for Platinum and they didn't bother to give it a good story. But remember, back in Gen I, none of the legendary birds were talked about that much in-game.
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As for people listing particular legendaries. It's extremely clear that they're just listing their personal favourites when they say Mewtwo sucks. Mewtwo was a necessity, even if you don't like its design or its popularity, it's
role (creating an Unknown Dungeon) is too important.
If you were going to make a list of the bare minimum from Platinum, it would have to be:
Mesprit, Uxie and Azelf (obligatory trio)
Dialga, Palkia and Giratina (title legendaries)
Heatran (Gen IV's Mewtwo, GSC got Red, RSE got nothing and we all saw how that turned out...)
Manaphy (obligatory cute obtainable out-of-game legendary)
Personally, I like the amount of legendaries (although I would have liked Cresselia to have an expanded story), but that's just the necessities.