use assertions in rspec

tl;dr -- you can do MiniTest::Assertions inside RSpec examples.

A few years ago, RSpec was extracted from the Cucumber project as a separate entity for BDD and testing with natural language in Ruby apps which may not need all of the weight of Cucumber. It gained traction pretty much everywhere, but it's primary evangelists were Rails developers (well, except DHH), and in the Rails community it was commonly used. But two years ago, DHH lit the spark that would become a major debate and one that "divides" Rails developers all over the map. DHH argued that [test_unit is all you will ever need][dhh] for testing a Rails application. And he's basically right, because you can rails generate model {something and then rake test and watch the assertions run.

why don't people use rspec?

RSpec is a pretty strange testing framework to work with if you're used to doing assertions and unit-testing each component of your application with the built-in Test::Unit library. Test::Unit, and it's successor, Minitest, was designed primarily as a unit testing framework at its most basic level. Since it's included in the Ruby standard library, it needs to follow the paradigms of how Ruby classes are designed by doing one thing, and one thing well. You're meant to add other libraries such as Mocha and Turn to give Test::Unit more functionality, or make it easier to read. These libraries all have their quirks, and tests written for basic Test::Unit tend to fail in strange ways when adding these other libraries on top of them, so typically users of Minitest or test_unit won't be keen on installing a lot of "sugary" plugins that could introduce a bug in the test suite.

the problem that rspec solves

If you completely divorce yourself from RSpec's syntax and the framework which surrounds it to provide a killer testing experience, you realize even less reasons to use Minitest. Speed and efficiency aside, RSpec gives you a lot of sensible defaults and configuration options just for modifying how its returning results to you. Don't like colors? Turn 'em off. How about the way it just shows you every test's name? Try --format=dot to make large test suites easier to read. Or better yet, use --format=progress for really big test suites where you need to see the health of all tests but don't need to necessarily see which tests fails immediately in real-time.

Tools like vim-vroom were built for RSpec. Gary Bernhardt's vimrc is designed for RSpec and works best when you're not constantly switching between Minitest and RSpec. It's more reliable to run single tests and single examples with RSpec, because you don't need a special shell alias or not use things like Turn which muck with the actual name of the test_method you developed.

have your cake and eat it too

You can take advantage of these lovely niceties of RSpec, without having to write in that nasty syntax! Since RSpec is just syntactic sugar overtop of Test::Unit, there's nothing stopping you from writing:

it "should pass" do
  assert true, "not true"
end

So why are you screwing around with Test::Unit and Minitest's difficulties and caveats? Just use RSpec. And make your life easier.