That's easy to answer. If you do this, FCP will remove every other frame of the 60p clip to match 30/24p. So if you change the speed again the frames are still missing from the 60p clip, they're not coming back.
What you can do is, keep the 60p and slow down by 50% without frame blending, you get basically double frames, but since the entire timeline is played with much more frames per second the visual result is the same.
But if you aim for 30p/24p as final result, you need to go for the Cinema Tools approach.