simply put, it creates dumb scenarios that make for good memories. Nothing like beating someone's best character with WFT while claiming "get fit" after making a strong connecting blow, crawling under a spammer's projectile game with a really dumb crawl, or landing Greninja's attack taunt randomly in a serious match. Did you know you can punt jigglypuff with Sonic's forward throw at around 150% with rage. Like he literally punts her into the background. You can't tell me that's not hilarious.
Its the same reason why I plan on picking Isabelle against characters like Ridley and Cloud. Pirahna plant just seems to have that level of dumb about him that I'm certain will make for some fun times.
So basically there's a crap load of ppl out there that just get off on troll picks and wreaking havoc for the hell of it. I thought it was funny how many ppl wet themselves over the novelty of piranha plant, yet 90% of them dropped him like a rock after the first week of release once they realized how janky his playstyle is. Honestly, when you think of all the actual characters that could have gotten in (Knuckles, Isaac, Geno, etc.) and instead we get trash like a nameless, faceless potted plant its just beyond pathetic.
I wouldn't be surprised if the plant fanboys were the same asshats doing the "Taunt Parties" back in Smash4. You know the ones, right? You'd go into an online FFA expecting to have a fun brawl only to find three morons spamming their taunts, preferring NOT TO FIGHT in a fighting game. You punch one and then end up getting ganged up on by all three. (On a side note, one of my absolute sweetest memories of Smash4 was having this happen to me, then rematching the turds with my main and proceeding to obliterate all three of them single-handedly.)
My point is, some people just want to watch the world burn in their stupidity and I really hope Sakurai and Nintendo don't cater any more to these types of ppl than they already have. The day they start adding tetris piece and "Labo-man" is the day we'll all know that smash has run its course. I know there's some fun and wacky stuff we can do with smash, and that's great to a degree, but I feel that the heart of the franchise is based largely on mashing up the most iconic characters in gaming and a random mob character (without even serviceable limbs, no less) doesn't really do justice to what could have gone in instead. Sorry for the rant, but let's not forget this is a fighting game. Not some meme-generating playground for trolling Otakus.