I think you'll find that many members here have a nice set of names in a particular niche. Maybe not as many as us 3, but I know lots of members who have 20-30% of their names in really niche fields.
I have a few niches that I'm involved in. I have some in geos, recycling (planet health would be the best description), energy sources (also ties in with planet health), and some with disease/illness. I am still building my portfolio though, so I maybe have 5-10 in each category.
I'm in the process to sell 90% of my portfolio and then refocus on 3 areas.
Transportation - it's a wide area, i'm going to focus on an area inside
Health related - Such as Osteology.US...
finance & business related - training, education and such
The ones that already fall into these sectors, i'm going to keep, the rest is going on auction...
Photography
Scuba Diving
Boats & Water Sports
Winter Sports
Aviation
Public Utilities
Geographic (cities, counties)
One word domains
.info domains
Small Business type domains