[PvK]

Most of them are useful, though not to everyone, and not always. Some of them offer some things that some nations don't have. Some are good because they are cheap or require few resources so they can be hurriedly hired and sent to absorb fireballs etc. Many of them are good enough that they're useful because of where they are, or because you don't have the gold or resources to only build your favorite units. Some of them are very good and useful to almost anyone.

If you want to build a lot of a particular one, build a fort with a good administration value at their location (then conquer all adjacent provinces if you want even more).

PvK