As a journey author, I want to run the journey without it being tied to a plan so that I don't have to resort to cumbersome workarounds in order to avoid distorting plan run results.
Currently, there is no way to run a journey in the cloud if it is not assigned to a plan. When it is assigned to a plan (or multiple plans), it runs under that plan (or forces you to select a plan under which to run) and also reports results to that plan. I find it difficult to see any benefit in this design decision. The downside of it is that whenever I need to run a journey, the run is reported under the plan, essentially diluting the report. Having to create some temporary plan and assign the journey to it just to avoid distorting the plan run report I care about simply doesn't make sense.
Long story short: Running a single journey as a standalone entity should be the default.