A battery occurs when a harmful physical contact occurs. Contact with a weapon is pretty much by definition battery.
The use of such a drug in the commission of rape or other violent crimes would then be a very easily proven case. If the substance is present in a victim's body, then battery has occured, basically by definition.
Given that rape and other violent crimes that are committed with the use of such drugs may not leave other physical signs on the body of the victim, then this may be the only physical evidence.
The examination for presence of such a drug in the bloodstream is also much less intrusive than for a typical rape kit exam.
There's nothing binary or black and white in such investigations, so this is an additional avenue to provide evidence to support the prosecution of these violent criminals.