I agree, Cassie, use my DNA profile to track down criminals. I think all services should allow you to opt-in (but have the value set at No as defaults) to A) will you allow police to compare your DNA to unidentified victims of crime and B) will you allow police to compare your DNA to unidentified perpetrators of crimes? I think lots would go for option A and then a subset of those would also go for option B.
I did a little bit of DNA work at uni, and am very sceptical of using DNA from these types of databases to fight crime. There are a lot of room for errors, from contamination of the sample, to false positives due to a narrow sample of markers. It is true that our DNA is unique, but you never test the entire genetic code. If you choose the wrong set of markers, you can have two very different people with the same identical test results. In normal criminal investigation, they figure out the physical and circumstantial evidence first, before testing the DNA. I'm afraid that it might skew the results to start out with a positive DNA test, since many jury members considered it as "truth".
It is the same with family trees in the online databases. When I searched for some of my relatives and ancestors, their trees were full of mistakes. We have a lot of people with the same or very similar names, born in the same area, around the same time. In some families, they would name 3-4 kids the same first name, to make sure that at least one of them survived and could carry the name forward. Often, you have to know middle names, nick names, or which farm they grew up on, to figure out which one you are looking for. My surname belongs to a village, so I'm only related to 1/10 of the people with my surname (even less when you count the other villages in different areas/countries with the same name). The people who are into geneology know this, and will always double check before they integrate work done by other people. But for someone just starting out, it might become rather confusing. And they might leap to the wrong conclusions.
It doesn't really matter if someone gets the family tree wrong, as long as it is used as an amusing hobby. But if it becomes part of a chain of evidence, that is a different matter. And, although I have a very high trust in the police and the legal system, they do sometimes make mistakes.