In open combat, a Weedle is a weak pokemon which poses no threat to anyone save an already exhausted trainer. Weedle trainers, however, do not use them for open combat.
Weedle also stand at a height of only one foot and possess the ability to crawl up walls, on ceilings, through windows - anywhere an assassin might conceivably need to go. The venom they naturally produce is not especially potent – perhaps strong enough to kill an equally small pokemon, but not nearly dangerous enough to kill a man.
Long ago the Koga ninja of Fuschia City learned how to replace a Weedle's venom with deadlier poisons, and despite a herculean effort of clan warfare and targeted assassinations, the secret has by now become common knowledge within the criminal underworld. Weedle have become the most common method of assassination in the pokemon world, so feared that a Weedle on one's ceiling is often used by criminal gangs as a warning to would-be opposition, who typically comply out of self-preservation instinct and terror.