In a competitive team, you want to use the best Pokemon for the job. Why would you ever use Manectric when Raikou is available? For this reason, the best set for any given Pokemon is sometimes not the most generally effective.
For instance, you might think that Vileplume's most powerful set is Swords Dance, Sludge Bomb, Sleep Powder, Razor Leaf, but most of the time, Victreebel does that better, so why would you use Vileplume to do that instead of Victreebel? If you give it a Sunny Day, Solar Beam, Sleep Powder, Hidden Power Ice / Fire set, why not use Exeggutor? Those sets may be powerful, but they are outclassed by other Pokemon with the same set, so there's no reason to use them on Vileplume, except in some rare cases ("I like Vileplume's better defenses on my Swords Dancer!"). However, Vileplume does have some interesting moves in Aromatherapy, which can be combined with other moves to make sure your Vileplume isn't just a worse Blissey / Celebi (you'd have a hard time justifying Aromatherapy, Leech Seed, Hidden Power Grass, Moonlight, as Celebi can use Heal Bell, Leech Seed, Hidden Power Grass, Recover with more effectiveness).
I'm just using this as an example, so if you disagree with what I said, then I'd just use the example of Suicune vs. Wailord, why would you ever give Wailord Rest, Sleep Talk, Surf, Ice Beam, when Suicune can do that set better? Sure, Wailord might be able to take a couple weaker hits with that set, but Suicune can do everything it can do better with that set.
Basically, what I'm saying is that I saw a thread a while ago in which people said you shouldn't take other Pokemon into account when determining a good set, and I say that's incorrect. If another Pokemon can do the same job better, why not use that other Pokemon? Roar is a good move on Pokemon like Skarmory, but why would you ever use that over Whirlwind? Sure, you can use Flame Wheel, but wouldn't a higher power Fire move do you more good?
/edit: Because this topic is so old, and because I am departing from the original subject matter, my responses to the comments can be read here: http://www.smogon.com/forums/showthread.php?p=1104937
For instance, you might think that Vileplume's most powerful set is Swords Dance, Sludge Bomb, Sleep Powder, Razor Leaf, but most of the time, Victreebel does that better, so why would you use Vileplume to do that instead of Victreebel? If you give it a Sunny Day, Solar Beam, Sleep Powder, Hidden Power Ice / Fire set, why not use Exeggutor? Those sets may be powerful, but they are outclassed by other Pokemon with the same set, so there's no reason to use them on Vileplume, except in some rare cases ("I like Vileplume's better defenses on my Swords Dancer!"). However, Vileplume does have some interesting moves in Aromatherapy, which can be combined with other moves to make sure your Vileplume isn't just a worse Blissey / Celebi (you'd have a hard time justifying Aromatherapy, Leech Seed, Hidden Power Grass, Moonlight, as Celebi can use Heal Bell, Leech Seed, Hidden Power Grass, Recover with more effectiveness).
I'm just using this as an example, so if you disagree with what I said, then I'd just use the example of Suicune vs. Wailord, why would you ever give Wailord Rest, Sleep Talk, Surf, Ice Beam, when Suicune can do that set better? Sure, Wailord might be able to take a couple weaker hits with that set, but Suicune can do everything it can do better with that set.
Basically, what I'm saying is that I saw a thread a while ago in which people said you shouldn't take other Pokemon into account when determining a good set, and I say that's incorrect. If another Pokemon can do the same job better, why not use that other Pokemon? Roar is a good move on Pokemon like Skarmory, but why would you ever use that over Whirlwind? Sure, you can use Flame Wheel, but wouldn't a higher power Fire move do you more good?
/edit: Because this topic is so old, and because I am departing from the original subject matter, my responses to the comments can be read here: http://www.smogon.com/forums/showthread.php?p=1104937
