I see the PaxMate, Spire and other products as a useful material that provides various benefits when applied sparingly.
For instance, you would like to reduce hard drive noise from being transmitted through the case. One approach is to shock absorb each drive individually. This often requires either exclusive use of 5.25" drive cages, or modification to 3.5" cages. However, if the cage simply bolts to the inside of the case, there is an alternative: shock-absorb the cage.
For adding a barrier to noise transmission through such surfaces, Paxmate or Spire or other foam-rubber products can be just the thing. It compresses, and it adheres.
I also use it to fill holes in the chassis to control airflow and pressure. On larger panels, it DOES reduce the reflection of noise off the panels. The affects on cooling are only important if you intended deliberately to use your case as a heatsink. If your cooling solution does not rely on case-as-heatsink, it is not a terrible loss.