I think that a Shuckle can be pretty useful... is one of these strange pokémon you don't expect in a competitive battle
imagine it with the EV split trough HP and Def/Sp.Def with this moveset:
Sludge Wave
Power Split
Earthquake
Double Team
the first thing you do is use Power Split to get (way) higher Attack/SP.Attack then some double team for the evasion and then Sludge Wave and Earthquake
You have to think about probability in the metagame. Assuming you're talking about singles, Power Split will almost never work as intended, and most competitive environments ban Double Team because you are then just flipping coins to determine who hits.
Poison is a really bad attacking type, especially for Shuckle who doesn't even receive STAB on it and Earthquake doesn't receive STAB either, but could be good for coverage with something like Stone Edge / Rock Slide, if Shuckle wasn't incredibly weak even after most Power Split.
Power Split would be more useful on a crippling wall set using Rest and Toxic. You don't want to depend on Shuckle for sweeping, because you're not investing in attack at all, and even after a Power Split you're unlikely to be too threatening.
There is very little Shuckle could actually force out too in the hyper offensive environment (mostly walls, or Choice Band Pokemon stuck on a resisted move), so it's going to be a tough go. Not to mention Shuckle is shut down 100% by Taunt, takes 25% health loss from Stealth Rock, is crippled by Trick, and is outclassed by many other tanks with instant recovery, or at least some sort of threatening move. Not resisting useful move types is just unacceptable in a game with such powerful Pokemon and moves now. Blissey / Chansey is really the one exception because of just how insanely how her HPs are. Just look at the decline of Snorlax because of the power creep.
And Shuckle must usually compete on a Sand Stream team to really have a place, boosting his SpDef 1.5x. However, there are many really threatening Rock walls you can already run that would be superior (Regirock in Sand Stream is no joke).
And if you're using that in Doubles, you're going to lose the majority of the time. Double Team is not a great strategy for success because for every one time it causes a miss the first turn, you're going to be hit the other three times, so you won't get a streak going ever.
'Strange Pokemon' usually don't work in the metagame, because everything is carefully looked at by a thousand eyes over years. The one time out of ten it works can be charming and fun to pull off, but if you're in a tournament, you wouldn't want to run it. There are standard Pokemon because they do work better than others. Some go up and some go down, but competitive viability is usually established early on. Sadly, Shuckle is a cool Pokemon, but it will never improve in the metagame unless it receives a big boost in evolution and move pool.
