I think these fall into the use case of #1877.

Yes, correct.
I did add it that way but I saw tickets opened for adding new search engines, that is why I though that it would be useful to have the url I provided added for everyone's use. I am sure that there are other genealogy fans out there.
(see for example #1844

Those don't fit the definition of a general purpose search engine.

Why not?
From their website I understand different. I do get many guests thanks to this service. It is a yahoo or google engine but then for familytrees only:

"FamilyTreeSeeker.com only indexes a so called Gendex file that remains on your own server. Each name in the search results will link directly to the family tree page of your website with the genealogical information about this person. So, FamilyTreeSeeker.com won't show off with your ancestral data, but brings visitors there where you want them: at your own family tree website"

Specialized audience.

The goal of #1877 is to provide a mechanism for user-defined search engines. An example plugin will allow users to enter their own personal list.