An interesting solution, but somewhat troublesome if you're running just one unit test that doesn't touch that backend service.
If it's retrieving data over HTTP, you can just use the awesome VCR gem: https://github.com/vcr/vcr It's awesome for testing HTTP APIs (along with webmock), since it works relatively transparently and makes your tests far less fragile.
Mocks can be useful for testing interfaces (APIs with a strictly defined behavior), but they have significant limitations. Mocks are stubs of a class at the time the test was written—it's very easy for them to be forgotten and no longer mirror the behavior of the mocked class. Additionally, they are code that increases the complexity of the tests making it harder to comprehend what the test is doing.
Starting up a separate system simply to run one unit test does seem like a hassle, but in practice it hasn't slowed down our tests one bit. The external dependency is started as a background process, spinning up at the same time rspec is bootstrapping. We also have a number of different tests that interact with the service, so the value is even greater.
If it's retrieving data over HTTP, you can just use the awesome VCR gem: https://github.com/vcr/vcr It's awesome for testing HTTP APIs (along with webmock), since it works relatively transparently and makes your tests far less fragile.