Depends on how they're using it. To find an unknown person and prove they were at a scene - yeah you'll have the 100 person's worth of DNA to sort through and then match against a (presently) incomplete DNA database. But if you already have a suspect and need to place them at the scene, if their DNA is one of the 100 then they have shown that.

But we’re they at the scene or did they just bump into someone or something that was there?

That's something that would have to be addressed at the trial by the defense attorney raising challenges.

If the DNA is present, it's present - barring any procedural mistakes by the forensics technicians (mislabeled sample, dirty lab equipment, didn't follow manufacturers instructions, etc). Or deceit by one or more members of the forensics team to implicate the suspect.

>That's something that would have to be addressed at the trial by the defense attorney raising challenges.

This is the wrong point at which to correct the problem. When prosecutors are allowed to introduce "science" as their evidence, jurors give this way too much deference. It's the prosecutor basically saying "you're too stupid to understand it, so indulge my appeal to authority" and jurors tend to happily acquiesce. This is why judges are supposed to gatekeep expert testimony and not just let any quack step in and make outrageous claims... and despite their attempts, quacks have repeatedly done just that these past few decades.

If is true that the presence of DNA means essentially nothing, then it shouldn't be permitted to be introduced at trial. Protection from wrongful conviction shouldn't rest on someone having an attorney expensive enough to be able to unsway too-easily-swayed jurors with counter-specious arguments.

Except DNA may not be present: the probabilities involved with partial DNA matches become very problematic.