Dark Samus is just a Dark Samus.
Play the Metroid Prime games, because you obviously haven't and they're great games.
Dark Samus would just be a Samus clone!
Dark Samus has actually built up quite a respectable repertoire of unique moves in the second two Prime games, and she could also have some abilities from her Metroid Prime form in the first game. The only moves she shares with Smash Samus are the power beam, missiles, and super missiles.
Dark Samus does not use bombs, the screw attack, or the grapple beam at all. Giving her those abilities would require an effort to have her as a clone; it’s like Ganondorf being a Captain Falcon clone, even though he has plenty of unique abilities.
Sakurai thinks Dark Samus would be a Samus clone, because Brawl's trophy description calls them identical.
If a character's trophy determines what a character has to be like in the next Smash Bros, why doesn't Pit look like this?
Even if she is different from Samus, why should we add her? What can she add to the roster?
Dark Samus can…
-Fly
-Turn invisible
-Become briefly invincible
-Shape crystal phazon into pillars and walls
-Boost herself at great speed in Morph Ball form
-Create shockwaves by launching herself into the ground like a meteor
-Make copies of herself and attack in tandem with the clones
-Spin right round, right round
-Corrupt people with Phazon, eventually turning the corruption into mind control
And more! She’s one of the most versatile potential newcomers for Smash, since you can base a moveset on nearly any combination of these gimmicks. It's ironic that so many people think Dark Samus would be a clone; if anything, an accurate Dark Samus would be one of the strangest characters in the game.
Dark Samus has Samus in her name. We don’t need three Metroid reps with Samus in their name.
Would it help if she was called Metroid Prime?
This is also a rather strange reason, since the three Samus variations would be vastly different from one another (Unlike, say, the Star Fox reps in Brawl... And people still want a fourth!).
Dark Samus isn’t important enough.
Dark Samus is one of the most important figures in the Metroid franchise, equaling Mother Brain and only being surpassed by Ridley. She is the main villain and final boss* in all three games of the Metroid Prime trilogy (Spoilers: Metroid Prime is Dark Samus). Metroid Prime is one of the highest-selling Metroid games and has the highest critical reception, and the latter two titles, while lesser successes, still got good critical reception. Metroid Prime sadly got little respect in Brawl (One stage and a few trophies, not counting the unrelated Metroid Prime Hunters game), but Dark Samus would be a great way to represent the series.
*AU 313 was merged with and being controlled by Dark Samus.
Metroid Prime is finished.
So was Mother. Metroid Prime is one of the most critically lauded main Nintendo series; it deserves love and attention.
Dark Samus is in competition with Ridley.
This is probably the greatest point against her. Ridley is unquestionably more popular and is probably the only mainstay character of the series outside of Samus and the Metroids. I would love to see him in Smash 4.
However, Dark Samus would be a great alternative if *SHOCK AND HORROR* Ridley does not get in. Sakurai saw the demand for Ridley in Brawl and still did not include the character, so there is always the chance that he may once again be absent. If Sakurai's opinion is truly static, he would have the same opinion he expressed before on Ridley: Maybe, yes, no.
Also, Metroid could have four characters.
No it couldn't.
The Metroid series is ongoing with 11 games. It has two characters.
The Mother series is long-dormant and has three games, only one of which was released outside of Japan. It has two characters.
The Star Fox series has half the games of Metroid and has been critically unsteady since Adventures; it received three characters in Brawl, and some people are even suggesting a fourth.
Metroid can have four characters.
What about Adam Malkovich/Sylux/Anthony Higgs/MB/Rundas/etc…?
Mother Brain can't work unless the Dev team gets very creative; her stationary brain form is just that, stationary, while her giant form actually does fit under the "Too big" argument leveled at Ridley, especially since Mother Brain's massive brain would be tiny if it were resized. MB would not be satisfactory; she's essentially a different character from a terrible game.
Because Mother Brain is not very well suited to Smash Bros, Adam Malkovich is the next Metroid character in line. However, I have no idea why someone would want Adam. He is one of the most hated Metroid characters because of his overbearing roles in Fusion and Other M, as well as his bland personality. He has no powers beyond a freeze gun and a bullet-firing (AKA not in Smash Bros) gun.
Anthony and Rundas have no importance to the franchise outside of their respective games and aren't particularly important even in their games. And Sylux? Don't make me laugh.
Why should Metroid get another character?
Why should any franchise get another character? It's not like they've been rational with character numbers in the past (LOL Star Fox having three characters). Maybe they should just cut everybody but Falco and make Super Smash Bros Hipster.
Dark Samus isn’t popular in Japan.
Metroid isn’t popular in Japan. Is this really going to get us anywhere?
Dark Samus can just be a Samus alternate costume!
She could. However, it's a bad idea.
First off, Samus has
tons of potential alternate costumes. The Fusion Suit, the vanilla Power Suit, the Dark Suit, the Light Suit, and the PED are all great-looking. Why introduce Dark Samus as an alt when there are so many better ones that are actually Samus?
Second, the costume would be weird and present issues that other alternate Samus costumes wouldn't face. As mentioned before, Dark Samus does not have the grapple beam, the morph ball bombs, and the screw attack. She also cannot shed her armor to become Zero-Suit Samus, since Dark Samus
is the armor. And let's not forget that any rendition of Dark Samus is incomplete without her trademark booming evil laughter!
Third, just because you can make something an alternate costume doesn't mean you should. Jigglypuff could be a Kirby alternate costume, but I don't think anybody in their right minds is proposing that idea.
Would Dark Samus as an alternate costume be a neat extra if she doesn't get in? Sure. Is that a legitimate argument against her inclusion? No. Being a potentially decent alternate costume choice shouldn't hurt a character's chances; if they're not going to get in, they're not going to get in for very different reasons.