[cabalamat]

> Why would someone vote for a law that seemed to them likely to create harmful side effects

Because as well as the bad effects, the law might also have good effects, i.e. be a net improvement. Most policy proposals have some potential bad side effects -- if they didn't, people would already be doing them.

So if a policy appears to be net-positive, it is probably worth supporting.

> rather than waiting for a less ambitious, better-designed law that wouldn't?

If a referendum fails, it may make it less likely that a better designed law will come along later. This is IMO the case for the referendum on electoral reform that will be held in the UK next May -- see http://en.wikipedia.org/wiki/United_Kingdom_Alternative_Vote...