Considering how things work on bitcointalk, do you really believe that an average member (who is usually hiding the fact that he has more accounts in signature campaigns) would join with one account only while being accepted with more? Honor system doesn't work around here and everything that can be abused for financial gain, will be.
In other words, we convict someone before he commits the crime?
If you refer to the average member as someone who does not care about anything to get money, scumbag-naim027 style, yes, but if he has some brains, if he is good enough to be accepted with both accounts in a campaign, he can join with only one and continue appying with the other to other campaigns and make everything legal, so he avoids a hypothetical future in which it is discovered that he is owner of both accounts and has been cheating, so he loses the possibility of earning money with both.