just a fundamental difference between midi and audio, Chuck, so not an appropriate comparison. Audio merge mixes the audio waveforms of two different instruments to a final track that is the sum of the different waveforms whatever the waveforms were. Midi merge combines the note and other data of two tracks. Since the final midi track can have any instrument applied to play, it can do things your audio merge cannot since once recorded your trumpet and sax in audio they cannot be changed to piano and strings. Each midi track can have one instrument, although you can change instruments through the timeline of the tracks as you go along, you have multitrack sequencers because it is easier to edit with one instrument per track. So it really is a question of apples and pears with different sets of uses and limitations with each method.
Midi merge has many useful functions, you can record left and right hand piano parts separately and then merge the result for instance.