Steve, I'm a Photo Mechanic user, in part because as a member of the press, my photos must include IPTC (International Press Telecommunications Council) information (It's not ITPC.), and because by including IPTC data with every photograph I produce, they are far better identifiable in case of theft, and because they are far better searchable.
The Nikon manager is considerably inferior to Photo Mechanic, and doesn't include a number of fields I'm required to send in, plus it is missing crucial fields such as "keywords" and "subject codes," which are required by every press organization I submit to and which are important as searchable fields.
The Nikon manager includes the following fields which are standard IPTC fields for photographs: Caption, Event ID is typically Supp Cat 1 for my clients, Headline, City, State, Country, Credit, Source.
Via Photo Mechanic I can fill in all the necessary field needed for press submission and cataloging which include: Caption, Headline, Keywords (I'm required to use structured keywords which Photo Mechanic can handle directly, but the Nikon manage cannot, and supplement them with extra descriptive keywords as necessary.), Supp Cat 1 which is used as an ID specific to the client, IPTC Scene Code, IPTC Subject Code, Intellectual Genre, Rights Usage Terms, Special Instructions (if any), City, Location, State, Country, Country Code, Date, Photographer, Credit, Source, Copyright, Copyright URL, Contact City, Contact Address is typically optional but it's available, Contact City, Contact State, Contact Zip Code, Contact Country, Contact email, Contact phone is typically optional but it's available, Contact Web URL.
When I copy my files from my memory cards to my computer, I use Photo Mechanic's ingest feature and apply the appropriate IPTC information automatically as I copy the photos, to the photos. In post processing, if any, I can customize the IPTC data via Photoshop as necessary, which is normally a change to the caption, and headline to make it more specific.
While the Nikon IPTC capability in the D4 is a nice start, it's really not sufficient for a press photographer, but it's a nice addition for cataloging for non-press photographers. It's certainly no match whatsoever for Photo Mechanic.