Its funny; everyone thinks PP is a lame, undeserving addition to the roster on account of being a generic baddy. But let's be objective here and take off our bias glasses:
-He is historically significant, being an antagonist older than Bowser himself and being a foe in the classic Mario Bros Arcade, making him a classic fighter in the same vein as G&W and Pac-Man.
-He has appeared in literally dozens of games as a recurring obstacle and foe. He is prevalent in both Mario and Yoshi games, and demonstrates a wide number of abilities, like breathing fire, breathing poison, eating Mario or Yoshi, growing into long vines, attacking from pipes in any direction, producing offspring, and spitting spike balls at the heroes. In fact, Piranha Plants have among the most abilities of any Mario mook, making them much more interesting than a simple Goomba or Koopa Troopa.
-He is a completely unexpected fighter that totally works and fits right in, yet no one ever saw coming; something Sakurai works hard to dish out every game whenever he can. He always tries to be one step ahead of speculators and make unexpected choices (R.O.B., WFT, Robin and Lucina over Chrom etc.). And with PP, he succeeded.
-PP is, being factual, blunt, objective and unbiased, more iconic than virtually every highly speculated fighter: Isaac, K. Rool, Shantae and especially Geno. Ask anyone if they know what a Piranha Plant is, and if they have even vague knowledge of Mario, they'll know straight away. Geno? Isaac? Banjo-Kazooie? Skull Kid? Much less likely.
-In a roundabout way, man-eating plants in general are a recurring theme in multiple Nintendo games and other franchises. People keep calling PP "a stupid plant" but the reality is that this trope of man-eating plants has also appeared in Pokemon (Bellsprout), Zelda (Deku Baba), Final Fantasy (Malboro/Ochu) and even the popular indie game Plants vs Zombies, which has appeared on Nintendo systems (Chomper). In a strange way, PP represents a bigger part of gaming history than we think about.
-PP looks straight up fun. Being honest, I think he looks more fun than I could ever imagine Geno or Skull Kid being. Entirely subjective of course, but there's more creativity and personality in his moveset than many other fighters, and he doesn't even have a face or proper limbs. He's amazing to behold.
In conclusion, Piranha Plant should stop taking flak for "using up roster spots". He is perfectly deserving, and Sakurai has a right to add his own creative ideas to his creation. The guy looks like he's wasting away from stress and is unbelievably tired. He gave us 74 fighters we wanted. My verdict is let him have the one odd fighter he wanted, and stop being greedy. There is no way to please everyone, so sometimes you need to do something fun for yourself; throw randomness at the wall and see what sticks. And that is Piranha Plant in a nutshell.
A year from now, we will all hail this character as a complete fit and a genius addition, once the salt has worn off. You can count on it.