Dead cells can cause all sorts of artefacts.
Dead cells have a naughty ability to nonspecifically binding antibodies.
The problem is significantly exacerbated when looking at relatively rare populations. For most of us, our cell preparations usually contain only a few percent dead cells. PI can be measured in a variety of different detectors, including the PE, Cy5PE, and other red detectors. If you have particularly bright stain in that channel, you can gate out PI-positive cells in a different channel. Therefore, you can't distinguish it from a typical immunofluorescence marker.