Why should she be playable? Edit
Because then there would be 3 Samus'. That would be hilarious. In actuality, she represents the highly successful Prime games, and she would be awesome. And she actually does shit in her games, unlike most of the other dark characters. Also, she isn't Ridley.
spoilers!!!!!!!!!!!!!!! don't read below unless you want them!!!
Dark Samus also is the Metroid Prime. So the Metroid Prime series is named after her. Damn, that deserves some recognition.
Dark Samus is currently seen as the most likely candidate to be an echo fighter. Her Assist Trophy has yet to be seen in Super Smash Bros. Ultimate, Samus lost her Dark Samus palette, and Ridley's victory theme, rather than being based on his theme like Dark Pit's, is a darker version of the Metroid victory theme (think Bowser and Bowser Jr.). While this isn't solid confirmation, it's more evidence than anyone else has.
add to this list if you support her