There are some people who simply have not registered, for one reason or another. Some also fear registration for their own reasons. In general, registration adds another step in complexity for elections. It is true that registration helps maintain the integrity of the voting process, but it also adds complications that could reduce voter turnout.
The short answer is that voter registration requirements lower participation in elections. Here is a good way to figure out what effect something will have on participation:
Political scientists say that people will not vote if the "costs" of voting are too high (compared to the benefits). Of course, you don't have to pay to vote so we are talking about other costs. Two of the major other costs is the time and effort it takes to vote. So if you have to be sure to register at some point before the election, the costs of voting go up and people will be less likely to vote.
So if you have to answer a question like this, ask yourself "does that raise the "costs" of voting or does it lower them? Answer that and you know what effect it will have on participation.