Of course collecting DNA of arrested alone is insufficient. Suppose someone has committed a crime, and they have not been arrested before - naturally, their DNA would not be in this database. So unless it's a repeated offence, the DNA base is pretty much useless to solving the crime. National DNA would be much better.
It is sufficient because until someone has committed a crime they do not deserve to be in the database. Plus it has been statistically shown that most criminals will commit another crime and therefore should be put in the database so that later crimes can be solved faster. Plus by putting all the criminals in the data base we could get a hit on someone who is not in the database if they are related to them.