I've just read up on SmartWater and it appears to be incredibly easy to foil: it relies on a "binary sequence" of different chemicals to create different combinations... so all the criminal needs to do is to walk into two or more shops before the crime to get coated with multiple overlayed sets of chemicals.
Example: Let's say there are just ten chemicals called A to J. Two shops each have their own unique set of chemicals:
Shop 1 has A B D H I
Shop 2 has B C D F J
Our cunning crim walks into both shops on purpose to get overlayed sprays and is now tagged like this:
A B C D F H I J
That's not going to stand up in court.
On the other hand, a DNA system will have genuinely unique and *unmixable* signatures.