The problem is that this code gets triggered after you have changed stereo view mode in the OSD, say from RENDER_STEREO_MODE_SPLIT_VERTICAL to RENDER_STEREO_MODE_SPLIT_MONO.
We switch to a non-3d display mode to match mono, and this code then switches you back to RENDER_STEREO_MODE_OFF which isn't wanted.

So far I've not seen what feature is lost with this code removed (on the Pi).
Perhaps you could explain how frame packed 3d breaks?