Having done this several times, over several years, I can assure you, you were lucky.
Not all Radios have the output to make this work the way you describe.
If the output is too hot, the attenuation provided by the music coupler is required.
If not, you can get away with it, fine.
Actually, on my own personal system, I have it connected the way you describe. (WITH THE STEREO SET TO MONO OUTPUT !! !)
However, on one I went on last week, it would never have been able to be used WITHOUT the coupler and its ATTENUATION capabilities.
Again, not all music sources are the same.
So be careful in advising everyone to just "BUILD YOUR OWN".