Почему там отдельный “unicorn_rails” для приложений для направляющих?

From the unicorn docs,

It is designed to help Rails 1.x and 2.y users transition to Rack, but it is NOT needed for Rails 3 applications. Rails 3 users are encouraged to use unicorn(1) instead of unicorn_rails(1). Users of Rails 1.x/2.y may also use unicorn(1) instead of unicorn_rails(1).

http://unicorn.bogomips.org/unicorn_rails_1.html

So, no, you don't need to use it at all.

It's worth pointing out that Rails only made the transition to Rack in late version 2 (I forget). So, when Unicorn first got released there were probably still a bunch of apps that had to be supported.