I think it's pretty clear that if you're faced with a prospect of armed conflict, should your opponents hire Friendlies you have to hire Dorsai or lose, and it's quite possible you can't afford Dorsai. So you wind up with a situation where being the first to hire Friendlies is very important, so of course people do.

(the complete lack of terrorism suggests the whole thing has a lot more rules than are made overt in the text.)

I think Dickson did a remarkable job of portraying the Friendlies as sincere believers without going into detail about what they believed. I found it surprisingly effective when I read these books.