More philosophically, when one's voting agency is shared with another person, as is the case in ballot harvesting, the opportunity for abuse multiplies. This is why most states have outlawed ballot harvesting.
Consider: I come to your door with a ballot. You had not thought about voting because you didn't particularly care about the candidates, but I am an agent for candidate A and because I got to you before the agent for candidate B, I convince you - good salesman that I am - to vote for someone you wouldn't have otherwise.
Thus the voting process become less about what the voters themselves think or care about and more about which candidate has the most resources to send agents out to ballot harvest.
The corruption that such practices invite completely undermine the voting process and it is only with a blatant disregard for the voting process - a fixation on power above principle - that a state would allow such practices.