There is another reason you might want to split a MEGX loop from stereo to mono. Say you split loop 1, loop 2 and loop 3 into mono loops. You'd now have loop 1, loop 7, loop 2, loop 8, loop 3 and loop 9. All six loops would be mono.
You could use loops 1, 2, 3, 7, and 8 for pedals placed in front of your amp. The routing is easy. Loop 8's return goes into the input of your amp.
Now you have one mono loop (number 9), and 3 stereo loops (4, 5, and 6) still unused. Here's where loop 9 comes in. You use its return to route the signal from another source into the remaining loops to place them in a second location.
Here is a 4CM use case:
First, convert all six stereo loops to mono, creating twelve mono loops. Route the front of amp loops as explained above. Now route the signal from the send of your amp's effects loop to loop 9's return. Now you have five mono loops in front of your amp and six mono loops in the amp's effects loop. Yes, you've sacrificed a loop (number 9) to implement this. But you now have on/off state control over all pedals regardless of where you place them.
Here's a second use case I plan to use with my WDW rig:
As above, I will convert 3 stereo loops to mono loops and place them in front of my Tone King Imperial II preamp. Now I use the send on my TKIP's effects loop to route the signal into the return of loop 9 on the MEGX. That places the remaining three stereo loops into the TKIP's effects loop. But I take it one step further. I already have a GigRig G3 Atom in that loop. So I add the three MEGX stereo loops to the Atom by placing the output of the MEGX into the return of a loop on the G3. The Atom's loops are all stereo, so the stereo routing is maintained.
This has the effect of adding three stereo loops to the remaining five unused loops on the Atom. Remember, I had to use a loop to put the three MEGX loops into the Atom's signal path. Note that these three loops can be added serially to the signal path or in parallel to the loops on the Atom. And the loops on both the Atom and the MEGX can be rerouted in any order. This stereo signal path is output to my Mesa Boogie 20/20 power amp and a pair of Mesa 1 x 10 open back cabs for my wet effects.
I use the "To Amp Return" output on my TKIP to send the signal from my front of amp pedals, and the preamp I'm using to my Fryette PS-2A load box. This is where I derive my dry signal path.