I wonder why they are swabbing and how they are clearing POI's without DNA?
New DNA techniques are developed semi regularly. At the moment, the may not be able to extract DNA or develop a profile. However, in a few years, as technology advances may enable law enforcement to draw DNA from a source they are not currently capable.
An example of this is the development of touch DNA, described here:
Touch DNA
"Even if suspects leave only 6-8 skin cells from the outermost layer of their skin, scientists can use those cells to develop a DNA profile."
So, that being said, while law enforcement may not presently have the DNA of the suspect, advancements in forensic science may enable them to generate a usable sample in the future.
That being the case, swabbing and collecting samples has 2 benefits:
1) Police can watch a POI's reaction. If a POI is adamantly opposed to a sample, despite being cooperative prior, or otherwise starts acting or appearing nervous, they have instilled fear and may look further. The opposite, as well, though to a lesser degree - a suspect who is cooperative and willingly gives a sample might give the impression that he's not their suspect.
2) In the future, if advancement in forensic Science enables them to develop a usable sample where as of now they currently can't, they can compare that sample to their current (then past) POI's without having to track them down or dig up their remains in the event that they died....
Finally, there is no reason to believe any suspect has been excluded by DNA evidence as far as I am aware. POI's can be tentatively cleared by a variety of means, such as corroborated alibis.