I can't see using more than one cataloging program. I think you need to commit ot a workflow that is either DNG based or NEF based and the answers become clearer.

Photo Mechanic at its core was developed for press photographers who need to download, name, caption, and keyword large numbers of images. Post processing is minimally included. With PM you can freely edit images with other tools and still use PM since it maintains the integrity of the original file. I can't speak to DNG specifically but it works very well with NEFs. PM is subsequently adding a cataloging feature in the upcoming release.
Lightroom came about as a cataloging program to organize files - especially different versions of related files. Having multiple versions of related files is common in a workflow that has RAW, DNG, TIFF, and PSD files for the same image. A catalog supports organizing and finding those files regardless of your folder structure. Lightroom has subsequently become a pretty good editor that allows the use of plugins designed for Photoshop.
Aperture is similar to Lightroom in that it is both a cataloging program and an editor.
If you only have one file - a NEF - which contains your versions, you probably don't need a catalog. But you may find other features of the programs or even a catalog are useful. For example, for HDR a catalog can make it easier to group images created at the same time. You may want a program for ingesting, keywords and captions.
If you use Lightroom, the main value of PM is for workflow - downloading and adding keywords and captions. For most people there is little need for both Lightroom and PM.

Eric Bowles
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