I think the most pointless pokemon is trubbish. I mean whats the point of haveing a pokemon thats a garbage bag and then when it evolves it is still pointless. A giant pile of trash along with the broken trash bag.
Chaosflame : In theory, Magikarp is the greatest Pokemon of all time. He knows splash, but most Pokemon trainers do not know that you need to slap the other opponents with the Magikarp. If you smash enough objects/opponents into submission, you will succeed.
I believe the most pointless is possibly Smoochum, it's completely useless, often forgotten, and quite frankly, is a frustrating road block that puts a happiness dependent bump in the road on a way to a useful Psychic/Ice type, Jinx.