There are certainly some spurious arguments, such as the typical prohibitionist canards that legalizing marijuana would increase the number of users or create more intoxicated drivers or lower economic productivity in the state. The oddest argument against Proposition 19 that I have seen recently is ridiculed pretty well in this blog comment:
So, Rick, in your opinion, is Prop 19 terminally flawed? Do the opportunities that the loopholes create for government to tighten controls make this initiative something that should be voted down?
You’re right, there are many examples where the INTENT of a new law/ regulation is about 180 degrees from the RESULT.